They also have effective lock-in. I don't buy movies on Amazon or Google play because I don't want to be locked in, or Kindle books without de-DRMing them, but Steam games? 80+ and counting.
Steam's monopoly is also why it sucks, I think. It seems like a textbook example of what happens with monopolies -- it produces an expensive product that could be much better and much cheaper.
Frankly, the publishers that have their own Steam competitors are worse. Steam at least supports Linux well and listens to players more than the publishers. I do wish DLCs were less rampant, but this is a gaming industry issue and less of Steam's wrongdoing. The GoG and Humble Bundle models are proven successes at this point, and I've easily spent 5-10x more on them than on Steam.