I used to agree with this sentiment. But now I think they engage in some nasty practices, like not allowing developers to list their games on other online stores at cheaper prices. That and they take a large cut (30 percent I think) Vs epic store at 12 percent as far as I can recall.
I mostly buy from gog because they give your the download to keep for life. Also I just like them. However getting their games to work on steam deck is more time consuming than just buying from steam
I'd be happy to find out that I've been misinformed but I'm pretty certain this is all true.