why are boxers and mma fighters do upright?

Just wondering. Every boxer and mma fighter I watch stands up so straight as if they were walking. I’m just wondering what is the reason to stay up straight in boxing? In other sports such as basketball or football, you usually get low for agility and speed. In boxing simetimes they do change levels, but most they are upright. Wouldn’t you want that athletic low position in combat sports as well?