There are a lot of properties that game mechanics can have that make people invest in games. Legible rules, clear feedback, deterministic and discrete cause and effect, clearly understandable win conditions and game states, being relatively simple to implement in code, having properties that make discrete variations and permutations of gameplay situations easy to build and easy to parse for players, setting rules up in ways that can be structured with real-time pressure, often embedded in large spatial structures to organically pace an experience...

Shooting (and combat more generally) has proven to be pretty easy to make satisfy most of these criteria. There are other core styles of actions that do as well (say, 2d Platforming, or clean puzzle mechanics like in games like Tetris).

These mechnical factors matter, because it's often the case that people who don't like violence in games would prefer games to focus on other kinds of challenges that they find more socially good in terms of morality or ideology. But then they stomp all over the mechanical styles of issues I was just listing above, and the results is predictably game designs broad masses of players don't want to play.

I've worked on both AAA hyperviolent games, as well as with educators on learning games with what they saw as pro-social game play, so this is a divide I've had front row seats to.

And to make what I hope is a productive contrast, one of the really great things about Undertale is that the designer didn't make being peaceful in the game lame. It is (or was for me) actively fun to try to figure out how to not kill enemies, because you still have to engage in bullet hell dodging while you try to psychoanalyze your opponents, and that dodging (for players who like those kinds of mechanics) still maintained a lot of the properties I just listed above.

To make a more real-world comparison, my father-in-law was an extremely successful junior college tennis coach, and he has noted in passing that he couldn't personally see how anyone could invest in Olympic sports like figure skating, just on the level of taking the competition that seriously. And his argument (he wasn't being universalizing, particularly, just tying it to his experience as an award winning coach) was that the extreme subjectivity of judge ratings was really offputting to him, as a competitor. Obviously tennis can have bad line calls and other controversial judge issues, too - all human sports can. But I think his argument ties in with my original one here; a lot of game players really like clean, legible rules with clear good and bad states so they can invest in getting good at games and take pleasure in their good play. And, as I say, shooting and combat at this point often fulfills that well.