Valve "played capitalism so well they broke it" and now Steam can't be replaced
(www.gamesradar.com)
It's also a matter of trust for me. Valve have earned my trust over the years by not being shitty, so I've stuck with them. Epic can give me all the free games they want, but I don't trust them. Same if some other company came out with a better store/launcher. If the company sucks, I don't care how good the app is.