It's my understanding that Steam does not really do exclusivity deals, outside of games they self-publish, which is why I asked the question. I wasn't sure if I was wrong on that point.

I've read some blog posts by indie developers that break down the cost model and effort required to publish to various platforms (Steam, Epic, etc). I can't seem to find them now though... But the general takeaway was that Steam is where the consumers are, so you end up making the most amount of money there. They also have a really nice pipeline that covers every aspect of development, publishing, and payment. I suspect the benefit of publishing on multiple platforms may simply not be worth the effort when Steam has the pipeline, the storefront, and more importantly, the customer base.

I've seen similar things in the mobile app space. The Apple store is where the money is at, and there are many apps that are not on Android. This isn't because of an exclusivity deal with Apple, but rather a business decision made by each developer.