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.
This is kind of like complaining that you have to own a Switch to play Nintendo 1st party games.
Portal is a Valve game. Steam is the PC launcher for Valve games.
FWIW, Portal was available on other platforms without Steam. I had my copy of the Orange Box for the Xbox 360 and that didn’t require Steam or a Steam account to play.