I have another answer: It's because true innovation is hard.
We have a ton of super-popular violent games to source ideas from for new games. We take an idea and modify it a little, and it's fun.
We don't have nearly as many super-popular non-violent games to source from. They exist, but there just aren't as many of them, and they're generally pretty "cozy" instead of pumping the adrenaline. Sports/racing games are an exception, but "non-violent" still depends on the exact sport and implementation. Many of them aren't non-violent.
It's the same reason that fantasy often still uses elves, trolls, and dwarves. They're really easy to source from, and coming up with compelling new races that aren't essentially the same as the tropes is hard.
Indies are into innovation. AAAs are into money.