There are some games tbat are sold on both steam and other platforms. Many of these games have external modding communities. Steam provides slick integration for “workshop” mods, which helps drive sales.

A few years back steam used to allow fairly open API access if you acquired the game elsewhere and just wanted the mods. And then they turned it off, but still allow direct anonymous downloads. A decision that only makes sense if they wanted to lock in any steam-adopted community to make going elsewhere difficult.

Turning off something people were using to avoid potential competitors is kinda exactly what would count as “monopolistic behavior” if Steam were to be ruled a monopoly by a court