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.
My point was, Epic creates these exclusivity contracts for games they fund. I think it's a net positive for there to be more art in the world, even if you have to wait a few months to buy them from your favorite storefront.
Valve doesn't fund the development of third party games. They do create some games in house (fewer than they used to sadly) but just like in Epic's case these are exclusive to Steam.
