That is debatable. For one thing, Steam is partly (mostly?) built off the backs of games marketing their games and providing a Steam link (marketing costs money for the devs). Steam kick started this chicken/egg problem by creating their own great games first.
Second, Steam does not provide your game any marketing (algorithmic visibility) unless it's already successfully marketed outside of Steam (marketing is not free), and again later once it hits a certain number of sales.
Third, per Tim Sweeney, games during the retail era had a bigger margin for the the studios than they do today [1]
[1] https://drive.google.com/file/d/19_NC1ZskeN47LHaYJziotbA0sqL...
edit: So I do feel a little upset that Steam gets free marketing for every game put on the site (important note you can (and should in most cases) place your game up on the site long before its ready to purchase, and steam will advertise other games on your page), doesnt provide any marketing in return (via the discovery queue) unless you bring in tens or hundreds of thousands of clicks, and then turns around and skims 30% of all my work which they are greatly benefiting from (e.g. what if the customer goes to my page, wish lists my game, then purchased a different game in the mean time? At least e.g. amazon has referral links)