The Steam TOS doesn't prevent this, but it's rather uncommon in PC games.
The main reason why PC and Console games do not have that same microtransaction-driven focus as many mobile games is because PC and Console games have a different audience. Mobile games generally are aimed at having quick sessions of a couple minutes while you have time available: toilet breaks, waiting for your bus, sitting on your bus,... . They have no upfront cost and as such need to recoup their investments through microtransactions. That's why a lot of mobile developers end up making their games start out fastpaced and then slow down rapidly, all the while trying to push the player to spend some amount of money on their virtual currency. They generally only need a very small amount of their playerbase to actually starting investing heavily (the so-called "whales") to get a large return on investment.
PC and console games, on the other hand, are designed to play in sessions of multiple hours: you have a lot of spare time and want to sit, relax and put in some time into your game. there is generally an upfront cost for these games because it has always been that way, where the player just pays around 60 USD to buy the game itself. However, because the player has already paid in advance, there isn't as much monetary incentive to put in those microtransactions, because PC and console gamers aren't as inclined to pay a lot above that initial investment.
However, this is changing lately. A lot of publishers are pushing their developers to create additional revenue streams. Up until recently, this was limited to having more expensive versions of the game people can buy, which can cost 150 USD or more and comes with tons of extras, with the most expensive versions usually containing a display piece like a statue or a coin. Then they added season passes and DLC updates, which sometimes are just cosmetic and sometimes add additional content. However, since about a year ago, a number of games have started to implement the mobile microtransaction-oriented Pay2Win economy that you mention, where the pace of unlocking things is deliberately slowed down and players can literally buy the things they want. Because publishers have added this IN ADDITION TO the at least 60 USD price tag the player has to pay to start playing at all, a lot of players are viewing this as price gouging and deliberately making games less fun so they can earn more money.
this is a currently an issue which is in flux. Some publishers are finding that they're losing preorder sales on upcoming games because their poor monetization strategy in recent games have put off players, and others have had to effectively remove some of their monetization strategy, either because of consumer uproar over pay2win mechanics, or because legal authorities have ordered them to stop because they're violating local laws involving unlicensed gambling. Time will tell what happens next.