Steam is doing well because they made a good platform, not the other way around.
Wrong. Steam is doing well because they invented a marketplace where other developers could sell their games with DRM and buyers could download them immediately from the comfort of their homes 20 years ago. They don’t need exclusive titles because they never had any competition.
Certainly having a good platform doesn’t hurt.
Gabe Newell even came out and said that piracy is not a pricing issue, it’s a service issue, which reflects Steam’s approach.
I don’t understand what that has to do with anything?
GOG came well after steam had established itself and, unlike Epic, they are not shitty
GoG is a side project of CDPR (much like EA or Ubisoft launcher) and I’m certain they don’t make much money, because very few developers want to sell their games DRM-free.