Official statement from Valve.
We shared with the NYAG that these types of boxes in our games are widely used, not just in video games but in the tangible world as well, where generations have grown up opening baseball card packs and blind boxes and bags, and then trading and selling the items they receive.
You’re right! We should stop that too!
But you don’t start 20 lawsuits for the same thing at the same time against everybody. You start with one case against one company, and if it rules in your favor, that sets stronger precedent to go after the others.
As for why Valve, I’m guessing it’s easier to demonstrate more specific examples of harm when you have a larger pool of consumers to draw from, and easier to get an American entity in an American courtroom.