Valve does some important gatekeeping that has a real impact:
There's (almost?) no ads and microtransactions in Steam games. If you look at mobile games or browser games, you can see that developers would put them in, if they could.
Fair point! Although that's a gate I very much like.
I don't think Steam has any complete blockers to microtransactions, given that there are games that do have it, although I don't think they're as predatory about it as mobile games are. Maybe that's down to cultural differences between PC and mobile, or maybe there's more too it.
Ads standing in the way of gameplay are indeed banned,[1] although there are ways around that I'm sure if devs really wanted to be knobheads about it, but people on PC tend to be more loud about ads in their games than on mobile.[2]
There's (almost?) no ads and microtransactions in Steam games. If you look at mobile games or browser games, you can see that developers would put them in, if they could.