Maybe they had an agreement with GOG? This is all personal speculation, but GOG was primarily known as Good Old (Ol'?) Games for a long time, as they would put that under their GOG acronym back in the day. It was essentially a storefront that primarily dealt with classics and keeping them available to consumers before they pivoted and started also focusing a lot on modern games. Maybe my memory is flawed and I'm completely misremembering the old GOG and they've always focused on modern games as well, so anyone feel free to correct me if that's the case.

Anyway, I wouldn't be surprised if GOG struck a deal with a lot of publishers for selling all their classics exclusively. On the flip side, it could also be that the publishers just didn't care enough about their old offerings to put any effort into porting them into other storefronts. Now that retrogaming is much more ubiquitous than it once was, some bean counter pitched this idea in a mid-quarter profit seeking brainstorming meeting and here we are.