For one: game developers might have extraordinary knowledge on their requisite game engines and shader programming but tend to loathe the type of logistical work required to deliver those games. Issues like managing and serving updates, DRM, and social features.

Steamworks SDK was one of the very first companies to offer that functionality pre-baked into their offering "free" of charge when companies chose to list on Steam. After a while, it became the default over time.

For many game companies, what usually happens is that initial investment is easy to go along with for the start when spinning up their game store (Origin, Bethesda.net) but eventually hard to maintain as it's hard to commit developers to maintain platforms.