There are several companies (Microsoft) who fall into the same category (being entrenched in PC gaming as a platform and intermediary between gamers and publishers). But Microsoft while more successful overall is not being considered to be a gatekeeper in this instance. They have further reach generally (they are the dominant platform for PC gaming as a whole, and have a competing game store). However you stipulate that Valve simply existing makes them a gatekeeper which runs afoul of the law put in place for economic reasons to provide a fair landscape. Why has Microsoft not pulled ahead of Valve? They take the same cut, have more exclusivity, provide more and arguably better hardware, have the Xbox game store and other competing services. They aren't being considered in this space to be gatekeepers (the two core platform services noted for them are LinkedIn, and Windows OS).

Nobody is forcing game development companies to do business with Valve. If they didn't (as an industry) they could absolutely exert enough leverage to push Valve off the top spot. Microsoft could almost definitively do so by themselves. They provide a great deal of the same services and products.

Valve only really seems to be guilty of innovating in a space that other larger companies ignored and being successful at pricing a product that people prefer. I'm not sure that's enough to warrant them being lumped in with companies that obviously use anti-consumer and anti-competition business practices to exert control over the digital market place.

The Digital Marketplace Act was created seemingly to force economic fair practice and provide a level playing field for businesses (startups or industry titans) to operate. Valve seems to be operating within those constraints and you haven't actually proven your supposition that they have done anything wrong to achieve what they have achieved.

Further I am going to say that you don't understand that "cornered the market" actually has a legal definition. "In finance, cornering the market consists of obtaining sufficient control of a particular stock, commodity, or other asset in an attempt to manipulate the market price. " - According to Wikipedia. So, how are Valve attempting to manipulate the market price of games?

We know already that they only enforce the price of steam keys (meaning that you cannot sell a steam key for less on any other platforms than you do on steam). But that's a steam key, and doesn't translate to the price of any other licensing key provided by any other license agreement.

What else are they doing that you feel or can prove is cornering the market. Getting to market first and offering goods at the same or a similar price as competitors with better service isn't it.