Isn’t being pretty the same thing as being pretty? That's right.
There is a big difference between being objectively pretty and being pretty.
However, the reason why they say, "Don't fix it if you're already pretty!" is that
you spend hundreds of dollars to make it a little prettier, and worry about side effects, and if you do get side effects, it will interfere with your daily life so much that you shouldn't do it if possible.
Going from pretty to pretty doesn't really change your life, and fixing one or two things doesn't make it pretty either, so why bother?
If you're pretty, I think it's better financially and mentally to just live without thinking about plastic surgery, so I absolutely discourage surgery if it's okay to a certain degree. I'm not saying you shouldn't do it because you're really pretty...
It's just that it's enough to live without surgery = Why fix it when you're already pretty? I'm just saying.