Because they don’t want you to. It’s not just for those reasons, those are just primary ones. They also don’t like the look of having it connected to a charge cable all the time, and users don’t “change the function” of anything on average so a solution that involves user choice doesn’t really work for them either.
If you’re looking for choice for the sake of choice when there is an obvious solution they can enforce through design instead, you’re looking at the wrong company.
You identified the issue right there, using the power button regularly is “normal” for similar devices. So how do they make it clear that it’s not “normal” for this device? Simple, make it hard to do.
I’m not saying you have to like it or even appreciate it, this is one of the most divisive things about Apple. I completely understand why people don’t like it and choose another solution as a result. It is the reality of how they design things though.