They do, though - very much so. Dominant market position equates to the same - it's called a natural monopoly. Just ask pre-1983 AT&T about that, or De Beers Diamonds. Both have "competitors" and both were (and are, for De Beers' case) monopolies. Valve also maintains a Most Favored Nations clause so they get the best terms with publishers across the board.
I would say that AT&T was a natural monopoly, because the barriers to entry involved in running telephone lines to every home made it impractical for competition to arise. It would also be an arguably pointless use of resources to run duplicate telephone networks across the country. Other utility suppliers are usually viewed as natural monopolies for the same reason. De Beers wasn't a natural monopoly, it was a 'normal' kind of monopoly achieved through its business strategy. I don't see Apple or Valve being natural monopolies and Valve isn't a monopoly at all.
I do think both Apple and Valve have dominant positions in their respective marketplaces of PC games and mobile apps, plus Apple has a monopoly in the iOS app market.
A dominant player in a market isn't necessarily a problem, but it can become one if that dominant position is abused, which is what Epic is essentially saying Apple is doing by forcing payment processing to be done via its service with a 30% cut. There's a similar argument to be made by Spotify and the like that Apple Music etc. also get an unfair leg-up by effectively not having to pay that 30%, although I'm not sure that this is central to Epic's case.
I imagine Valve is of particular interest to Apple because Valve operates on an open platform with a reasonable amount of competitors (GoG, Epic Games Store, Xbox, Origin, Uplay, etc.) and still charges something similar to Apple. Apple would love to be able to say that 30% is about the 'market rate' for similar services on an open platforms so adopting it on their closed platform (to the exclusion of competition) is not an abuse of its position.
A problem with this for Apple is that Epic's cut on its store is 12%, which suggests competition on an open platform can drive down prices for developers.