Apple puts the charging port on the bottom because they want to make the upper surface as smooth as possible. They certainly wouldn't tolerate anything as bulk

Apple puts the charging port on the bottom because they want to make the upper surface as smooth as possible. They certainly wouldn't tolerate anything as bulky as a battery door on the upper surface. And designing a battery door for the bottom, which also wouldn't interfere with the smooth motion of the mouse... it's probably something Apple engineers could pull off, but it would take a lot of work. Combined with the other advantages -- less waste, the fact that charge ports are ubiquitous, fewer moving parts -- it's no surprise that Apple, at least, would prefer a built-in battery. As for everybody else, I dunno. It may literally just not have occurred to them that some people would prefer it.