That convenience is everything. Doing it well, and not falling into the trap of putting profit (too far) above users is the challenge that is too hard for other players (except maybe GoG) to get right. It's like WiFi. You go somewhere, connect, it works, and then you don't think about it unless it's surprisingly fast, or there are problems with it. Everyone else's offerings on this space just feel janky and liable to take your money for some reason. Steam, for the majority of its users "just works". That's not to say there are zero buys with the software and that nobody has valid complaints about it, but just that in general it's great.
It's still facing the headwind that a lot of people still don't believe that Steam can give you a lean-back experience which is fun like a game console. Some people still think PC games all have sweaty keyboard and mouse control schemes and those crappy huge joysticks from the 1990s that were always falling apart and had to be recalibrated every few minutes -- and that's what is keeping the PS5 alive.
