But they do give you an advantage. If steam didn’t exist at all, without a comparable replacement, it would not be possible for you to move a real quantity of units at all. The market they provide has massive value, and their market share is a product of genuinely being far and away better than any alternative.
People don’t refuse to buy games on Epic or Origin or Uplay just because they need everything in one place. It’s because all of those platforms are so much worse that they degrade the experience of games purchased through them.
This is an anecdote, but it is also absolutely not speculation. I won’t install Epic, I avoid most AAA launchers/required accounts, prefer GOG, and get most of my games on Steam. Epic and many other studio launcher apps are hostile to the consumers or just a royal pain to use. I have a couple Sony games. Why should I have to be online to play a 20-year-old single-player game that I bought through Steam? So now I check if they have that garbage before I buy them through Steam.
I think Steam could afford to charge less, but I don’t think most smaller companies could get a basic store up for less than they charge (and the big companies have the tools to determine if thos is saving them money), and that still doesn’t get you everything Steam brings to the table, consumer confidence being the most important.
Again, that’s because every other way to distribute games is terrible.
And it doesn’t really matter, because any sales you actually drive yourself you can give them 0% of, with free steam keys. Sales through their storefront are inherently partly driven by their value add.