Maybe "I'm busy" means "I don't want conversation" or "I just want to be on my own" but saying that would come across as ignorant, dismissive or that there's something wrong which itself will turn into an hour of "look, nothing is wrong, I just want to be on my own, leave me the F' alone". Perhaps they just feel drained, overwhelmed or they simply just want to sit in their underwear eating cake and playing games. Maybe it's something really personal they don't want to open the door to further questioning with. Perhaps they're just having an absolutely awful day and want to sit and have a bit of a cry on their own. Whatever somebody is "busy" with is absolutely none of your business and absolutely nobody should have to tell you more than "I'm busy" if they don't want to. You're not entitled to a detailed answer in the same way you're not entitled to a conversation on demand.
I hate this modern entitled mindset social media and instant messaging has brought. Everybody has a device in their pocket now so everybody is expected to reply instantly, instant gratification on demand. The main thing I hate though, that these apps now tell you that a message has been seen. Everybody hates being left on seen but do you know what I hate far more? Sitting around doing absolutely nothing enjoying how comfortable my ass is on the couch because I'm totally drained from work and then I pick up my phone and curiously read a message from somebody. Now what do I do? Do I just leave them on seen? I'll tell them I'm busy. They're asking why, what do I say? I can't tell them I'm busy doing pretty much nothing because that sounds like I'm just being ignorant and dismissive with them, which I am, which I would be with literally anybody at that moment in time and I should be allowed to be because nobody is entitled to my time if I don't want to give it, especially if I'm in my own house, my 'safe space' where I shouldn't feel forced to do anything I don't want to do. Now they're asking me to do things and I have to tell them I would literally rather sit and do nothing right now than hang out with them which sounds awful but it's true. Now I feel like an ******* because I just don't feel up to dealing with social interactions right now but I've either been forced into it or forced to be ignorant all because it tells people I've seen a message so they're expecting an instant reply. Even if it doesn't say seen, "I sent it at 5PM! Why haven't you replied? I demand answers! Give me attention!"
If I tell somebody "I'm busy", it means I'm busy. If I don't tell them what I'm busy with, it's absolutely 100% none of their business BUT they can and should take it as "I've seen your message and I'm replying to you so you know I'm not ignoring you, I'll message back properly when I get around to it and have finished doing whatever it is I'm busy with". It's not a poor excuse, it's an answer to something the person either doesn't have time or didn't want to be doing in the first place.
Now ghosting, that's different, hence why I opted not to quote that part of your post.