Steam has multiplayer integration so you don't have to connect by IP to play indie games, that is massive. So many people either don't have access to their router or don't have the skills to configure it to play multiplayer without steam without having a server middleman which most indie games wont have.

Then steam reviews are the most accurate reviews there are for how likely you are to be happy with the purchase. I am much more hesitant to spend money on a game where I can't see the steam reviews for, so there is basically no way I'll buy a game on epic store that doesn't exist on steam since I am basically buying it blind.