Why can't I just do push ups to get an amazing upper body?
Push ups mainly will focus on chest, shoulders and triceps. But your body will get used to your own body-weight
The way we grow is when our muscles are contracted at an intensity that they haven't felt before, they will then be forced to adapt.
If you only do push-ups, that adaptation process is very unlikely to happen.
To get an amazing upper body you will need to start lifting weights.
Follow a proper training routine, start eating more and your muscles will grow accordingly