Because on moder hdpi screens, text rendering involves pushing around a lot of pixels. Especially if you do stuff like having translucent windows, aliasing, complex font rendering, smooth scrolling, etc. So, it kind of makes sense to use the GPU for this. Of course on OSX, much of the desktop is already hardware accelerated so it is easy to overstate the impact here. Apple fixed this when they launched OSX in 2000 or so.

I've been using iterm for years; when they introduced their metal rendering backend the impact was very subtle but noticable. Anyway, given the amount of time I spend using cli stuff, it kind of matters.