Why?

Combat is a central mechanic for most DnD games. It allows the players to use make interesting choices and see how they play out. Players get to use a variety of skills in order to achieve success. Their shortcomings are also a liability in combat, making their development choices meaningful over time.

You could reduce combat to a single roll, simply add up all the players to hit modifiers vs the enemies, make a single "combat check" and that's that. That wouldn't be interesting at all in comparison and yet we essentially do that for skills.