Except Valve allows people to sell their own keys without Valve taking a cut. That system is why at least half the vendors here can exist at all. The dev/publisher cuts a bunch of their own Steam keys and dumps them off with these shops who take less than the 30% cut that Valve takes from sales within Steam and the dev/publisher gets to keep the difference or pass that on to the customer as a discount. Steam just isn't a monopoly. They allow sellers to use everything that their platform offers for nothing more than a percentage of what is made exclusively from sales within the platform. A seller can sell their game through their own website and take home 100% (less whatever their payment processor charges, usually a single digit percent) of the sale, while still using everything that Steamworks and Steam in general is bringing to the table and all without any lock-in or requirements that they stick with Steam. All of that is a HUGE strike against considering Steam a monopoly, but that's not even everything.
So far as "the competition does not matter" that's largely because the competition (Primarily talking EGS here, but it's apropos UPlay and Origin too) hasn't done anything to make for a better value proposition other than paying for store exclusives and giving developers a rightfully higher cut of sales for a shot at a much smaller portion of the PC market. If Epic offered answers to Proton, Steam Link, communities, workshop, meta-games in the store, quick UI, marketplace, etc... It might make for a real challenge to Steam, but as it stands now there's nothing in EGS that puts up anything approaching half of what you get for the same games in Steam. I've never bought anything in EGS, but trying to use their app with any of the free games that they've given me has immediately turned me right around and sent me back to Steam while it takes literal minutes for the app to get me signed back in and going (and often has to spend time updating a game once it does) while Steam was good to go 2 seconds after boot, never needs me to reauth once I've signed into a system, and keeps my games updated silently without my having to notice or worry.
Now GOG on the other hand, where I have spent a decent amount of money and own a good number of games has managed to make a proposition of giving me a barebones store that gives me barebones downloads of games that don't need updates, or a launcher, without any DRM so I can just download an EXE and get my games. They matter, they're bringing something to the table that nobody else does and I love them for that. I go out of my way to buy games on GOG when they're the sort of things that don't need any of the stuff that Steam is providing.
If any of the other publisher owned storefronts tried to do anything half as ambitious as GOG or Steam, they'd probably matter, but the fact is that they won't because they don't think like Valve or GOG, they think like MBA shitlords who's single trick is extracting rents for properties made by smarter people, often back in the days before those MBAs knew their multiplication tables. The same school of idiots who saw how Netflix had a really good thing going and thought that they could have the good thing themselves so now we have a worse situation than we had before Netflix destroyed the cable industry and we get all of these platforms that don't work as well as Netflix did/does where you have to go to 30 different shitty places to get what you used to find in one really good place. A whole lot of idiots who paid a lot of money to learn in fancy schools that you can personally get rich by convincing a company to kill the golden goose, so long as you immediately proceed to get out of town so it's the next guy's problem to solve before anyone notices that the golden eggs aren't rolling in anymore. It's incomprehensible to me that people are going to bat for those muppets when all they ever do is make things worse so they can line their 401k with another million.
Except, legally in the US where Valve is based, you've got 0 legs to stand on.
Valve does dominate the market they're in, but they do so without creating an unreasonable restraint of competition in that market. They are dominant by providing the best product, not because they have unfair business practices which burden the competition. Like I said, Valve will literally allow game makers to go and take 100% of every sale they make (assuming they can process payments for free) while still allowing them to use the platform Valve have built and pay to maintain so long as they'll pay Valve a cut for the copies that are sold directly through the Steam store. Valve allows their competition to sell games that package said competition's stores inside of those games. Every EA or Ubisoft game comes with the competitor's store bundled in. They create tools that allow their competitors games to run on platforms that the competition doesn't want to bother with and they give them away. HOW IS ANY OF THAT AN UNREASONABLE RESTRAINT ON COMPETITION?
Steam is the antithesis of anticompetitive, they're not the single seller of any good beyond "Valve Games" of which there are now 22(?) among millions of PC games, and they don't generally dictate prices in the market; which is the succinct way of saying that they don't live up to any portion of the legal standard for what constitutes a monopoly. Give me something factual that implicates Valve as a monopoly or get out of here with this nonsense.