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Epic Games Store is offering developers 100% of revenue for six months of exclusivity
(www.videogameschronicle.com)
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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.