Because it’s a petty issue that can be in violation of human rights. Do you want to see people get fired over stuff that has nothing to do with the business they work for, like something they did over 20 years ago or something they did in private? That could upset the economy, and it’s all because of a stupid issue. Todd Snipes got fired for calling Trayvon Martin a “thug” (which is actually true), and said it on Facebook. James Damore got fired from Google for denying gender discrimination as a reason why there aren’t many women working for Google. And I already mentioned that Cinco de Mayo thing in the OP. Whether some everyday words are offensive or not because of a minor context not everybody pays attention to or because it’s not inclusive enough is a petty issue that taking a big deal out of it is ridiculous, but it has gotten well out of control before Donald Trump announced his run. I understand why the left wing supports it, but I’m not going to support it or even be friends with people that support it when it’s a job killer, goes against the meaning of common activities and morals, and is a double standard.
I guess I'm wondering what your definition of political correctness is? I would personally consider myself in favour of political correctness most of the time, but all that means is that I think that it's the morally responsible thing to do to avoid language that dehumanizes marginalized groups. But I don't always think people should be fired over "un-PC language", it depends on the application. If someone who is a relatively anonymous member of a company says something controversial on the internet, that probably doesn't warrant being fired, but I do think that if someone is a very public figure and represents their company, it should be taken more seriously. An example is that, I recently heard a story about a police officer who stated that he thinks gay people don't deserve to live, in a public setting. I feel like a police officer stating that he doesn't think certain members of the community should even be alive will make civilians feel unsafe, knowing that there is an armed homophobe in a position of power, and the whole point of police officers, whether or not they do a good job of it, is to try to make the community safer. Or in the case of someone like Roseanne Barr, who got kicked off of a show she was in over some racist Tweets, her entire career was based around being liked. Being a celebrity is about being liked. If you do something that makes you unlikable, that will inevitably impact your career, if the whole point of your career is that people like you. It's important to remember that, when you work with or for a company as a public figure, you represent that company, and are associated with that company, and if you do something that makes you look bad, it makes the company look bad, so I believe companies should be allowed to fire people that do and say things that make them look bad. I also think it's important to note that political correctness is not the only thing responsible for people losing jobs. Bigotry makes people lose jobs too, and a society that is overly-lenient on letting bigots get a free pass to say whatever they want with 0 social repercussions results in people losing jobs, too. Bigoted employers turning down employees based on their skin colour, or places where people are legally allowed to fire employees for being gay. Political correctness is not the only thing that gets people fired over personal things unrelated to their employment, bigotry does that too, and while it may not always be perfect, political correctness mostly exists because people want to fight against bigotry being considered okay and normal.
Personally, when I say I'm pro-political correctness, I don't mean that I think everyone should be fired if they say something messed up- I really do believe that the only times when you should be fired for saying "un-PC things" is when either 1. Your job is being a public figure and intrinsically linked to people liking and trusting you or 2. You are making the people in your workplace or community feel unsafe. But what I do believe is that we should live in a culture where people actually have conversations about words that hurt people and that saying "hey, this word hurts people" or "hey, this thing that happens a lot in society might hurt people" shouldn't just be met with "oh my gosh, shut up, cringy SJW!!", it should actually be given the respect of a discussion. Most of my "pro-political correctness" is not related to the law, it's related to people evolving how we talk to each other, to not think words have 0 meaning and that you can say whatever you want without it having any weight. There are even some cases where people have said extremely "un-PC" things but I still stand with them because I think the punishment they received was too harsh, like for example, Tyler, The Creator, is a rapper who said a lot of really, REALLY messed up stuff on his earlier albums, stuff that I find kind of sickening- but I don't think he should be banned from entering Australia over rap lyrics.
Basically, I guess I agree with you that political correctness isn't good when it gets to the point that non-violent language is met with a violent response, or when people have their rights stripped away over one offhand comment or something, but I also believe that we shouldn't pretend that words can't hurt people and should be allowed to have nuanced conversations about that without being put down as "SJWs", and I also believe public figures have the responsibility of maintaining their public image if they want to keep their status as a public figure. I don't necessarily believe people should make someone get arrested over an off-colour comment or whatever, but the right to be angry in response to ignorant remarks shouldn't be made to look like a crime either.
As a side note, I don't know every individual instance you are bringing up, but I looked up the Todd Snipes thing, and it appears he was a Beach officer, a type of law enforcement. In my opinion, law enforcement is absolutely one of those jobs where your public image is important. If your job is to make the community feel safer, then your image should reflect that. Making a disgusting comment about an 17-year-old black kid getting shot is, in my opinion, something that could make members of the community feel unsafe. If law enforcement or another authority figure makes their community feel unsafe, that should be grounds for firing IMO, even if it is "just their opinion" or "just their free speech", ESPECIALLY (but not limited to) if they make it public- social media is public. Saying something on social media is not equivalent to saying something privately, behind closed doors. "Thug" may technically be defined in a dictionary as a violent person, but in society at large, it is getting increasingly more racially coded. Even if the word "thug" didn't have racist connotations, in general, making a joke about a minor getting shot is something that I don't think most people feel comfortable hearing from someone whose job is related to public safety.