I'm guessing its simple, blind evolution. D&D and such had kill-and-gather-loot mechanics, and were pretty popular. So many offshoots of that genre got written and sold. Evolve some wings and fins etc and we are where we are.

If we actually decided where we wanted to go with video games, we could get somewhere. Til then we're going to wander around in this violent killfest wasteland, upgrading the graphics on the explosions.