The Epic First Run programme allows developers of any size to claim 100% of revenue if they agree to make their game exclusive on the Epic Games Store for six months.
After the six months are up, the game will revert to the standard Epic Games Store revenue split of 88% for the developer and 12% for Epic Games.
The problem is that none of these other launchers offer features like Steam Input, Proton, in-home streaming, a good overlay, and the Workshop. Steam competes by making their platform the most attractive to customers.
Alternatives to Steam need to find their own niche. GOG is doing well in their niche of fixing up older games and selling them DRM-free. The only “killer feature” EGS has is that they take a smaller cut from publishers. But end users don’t care about that, because it doesn’t translate to lower prices. I can chose between spending $60 for the same game on Steam or EGS, but the EGS version comes without all the extra features I listed above.