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.