Valve makes it very very easy to be a PC gamer, and, importantly, slightly harder to be a PC gamer who buys games in places other than Steam.

Yes you can buy games on GoG or Epic and play them on a steam deck or a steam machine. But it's juuuuust enough faff to be annoying enough, that you'd rather just get them on steam. I know people (and am a person) who have rebought games they already own so they are on steam, to make playing between steam deck and desktop more reliable.

It's the same with the steam controller. You _can_ use it with games outside of steam, but it's enough of a faff that you find yourself avoiding it.

It's incredibly effective, and why they are an effective monopoly in PC gaming.