No one is perfect in every subject.

Good thing you only have to choose a single major and non-major courses are typically easy intro courses. Like... I have a 4.0 in math. I can guarantee you that if I were a history major or a literature major I would not have a 4.0. I don't have to do that, though, so I'm all good.

There's also the fact that people who are good in one thing tend to be good at other things, especially if those other things are cloely related. For example, I most certainly am better at some areas of math than others, but in then end it's all math and I'm good at math. The other STEM subjects also require the sort of thinking that's required for math, so it's not hard to do well in the other related subjects. The hardest STEM class for me is biology, which I never had to take in college, and it's hard for me for the same reason history is hard for me-- too much memorization.

So yeah, I'm not perfect in every subject, but choosing a major based on your strengths makes a 4.0 realistic for lots of people. Also I have a lot of free time and use all my free time to study my ass off.