Something like this is on our road map, but we want to do it right. There are some issues with simply doing as you say, adding a scroll bar, and calling it a day. For one, you don't really want to encourage people to have massive signatures with tons of media in them slowing loading times, just simply because they can have a large one with a scrollbar. There's also the issue that spoilers are nice (and extremely common) for including things and the interaction of using spoilers inside a scroll-bar area could be annoying as well. Of course there are solutions around these problems too, but hopefully you can see why it's not as simple as it might seem.
same