People who bring in more sales are worth more money it's not shocking that they get a better deal. It's like saying capital one gives air miles to people who don't even need them. Well ya. It's not a gift it's purely mercenary.
Then they should lower rates across the board. There used to be a time when "just being on Steam" justified the high cost to smaller developers, but the market is over saturated now because they don't curate anymore that it isn't the case.
So where would a small developer launch nowadays? As far as I can tell, they're still on Steam, so they disagree with you that the cost is not justified.
Sony has been friendly to indies during the PS4 generation. Nintendo was especially good for indies in 2017 and is practically giving away development kits. Even EA is becoming increasingly friendlier to indies on their Origin platform.
Steam has brand recognition for developers, but that is changing with competition.