You know how people say regarding Epic/new storefronts "Oh well, it's just a launcher. You click a button, what's the big deal?"
On a whim I wanted to see if I had Peggle in my Origin account last night (incidentally my Origin/EA account is the only gaming account I've ever had stolen, and it was taken by some Russian dude to play Battlefield Heroes, lmfao). So I boot up Origin, and I realize that on my two and a half year old laptop, I've never logged into Origin. Now granted, there's not that much stuff in my account I want to play, but still. Almost three years without opening the client once to even log in!
I get that it's not a "hassle" to open a launcher, but the reality is that I just don't. Most of my library is centralized in my Steam account, because that's convenient and that's just what I open all the time. So I always remember what's in that account. It's the same reason why I my console gaming is primarily on my PS4 instead of my Switch. I turn the console on all the time to play games and watch movies/TV, so it's a central device in my life. My Steam account is the same way. Aside from missing features which suck, sure it's no big deal to "open the client," but more than anything I just forget to do it! Why would I want to have games fragmented across more launchers that I'll forget I own?