The biggest difference compared to the likes of EA and Activision is that the items have real monetary value. In Activision or EA games the money spent on the lootbox is effectively wasted. You generally can't trade the content to recoup the costs and in some cases where you can those trades tend to happen for in-game currency which again, has no real monetary value. There's no monetary incentive to "gamble" because from a monetary point of view it's always a loss. The lawsuit actually makes a case that with Valve there is a monetary incentive to "gamble" because the value of the item you receive from the lootbox translates to real world money. And while the lawsuit has to take sort of a roundabout way of proving that, I think most people agree that the stuff you pull from Valve's lootboxes has real monetary value.

The second point is supporting the infrastructure to make trades happen. One of the things that separates trading cards from gambling is that the cards themselves officially have no value. You can buy a pack of MTG cards and get something really valuable but you can't sell that back to WotC (Wizards of the Coast, the makers of MTG) and actually WotC doesn't even acknowledge that you pulled something really valuable because they claim all the cards have their value based on rarity and not the card itself. But it's valuable because there's a secondary market that has nothing to do with WotC. That's not entirely true for Valve because all trades happen on Valve's infrastructure and Valve literally shows the real world value of the items. Now Valve gets to claim they don't set the price, the market does, and that's not an acknowledgement of the value, but Valve still benefits and supports that market which itself is a vessel for gambling. Worth mentioning that it's not an argument that is being made in the lawsuit (at least I didn't notice it there) so it's less relevant or even completely irrelevant from a legal perspective, at least for now.