Slot machines are regulated because they fall under the category of gambling and video games don't meet the legal definition of gambling. (Although, I'd agree that it's starting to seem more and more as if they should.)
There's a pretty good explanation of the criteria at https://digitalscholarship.unlv.edu/cgi/viewcontent.cgi?arti... but the brief summary (from the article) is "Gambling consists of three elements: consideration, prize and chance. If any one of those three elements is missing, the game is simply not gambling (Rose, 1986))."
By looking at those criteria, there's a lot of ways video game companies can defend themselves. The strongest one IMO being that, from a legal standpoint, there is no prize. You "win" things or get drops but the TOS of all games say that the player does not own them (or anything else in the game) in any legal sense. The analogy would be more like a carnival ride where you are paying for the experience rather than the expectation of getting anything material in return.