One answer is quite simple. Not to sell loot boxes.

I mean counter strike and team fortress 2 worked fine and were extremely fun games before they added a virtual slot machine to their games. “they’re just skins” right? If they were given out for free in game it wouldn’t impact the rest of the games experience.

Valve can also prevent the sale of real world money for these items. Especially if it’s been flagged for Gambling.

Or as another stated disable or moderate the usage of their own API on these gambling sites.