Casinos are different because you can go from Cash->Chips->Cash all inside the same establishment. The casino is directly responsible for the control of taking and giving you back actual, real money.
Likewise, if Valve had taken cash, gave a loot crate, and then gave you the option of selling the item from the crate and getting actual cash back, then that could be pretty reasonably considered to be the same as a casino.
But, what arbitrary articles people decide to sell or trade to others for the potential of profit doesn't fall under the same scope if they do it completely separate from the original goods provider.
By that logic, any and every instance in which you "Pay money for a thing, don't get exactly what you want, and then trade or sell it", it becomes gambling.
At that broad scope, the used games market could be considered gambling. You pay money for a game, but since you don't know that you'll like it or not, you try it out, hate it, and then either sell/trade it to someone else, or trade it back into the store to get credit to pick up another game. Same thing for used cars, and so on.
You can't reasonably hold the original provider of a good accountable for the end user's secondary activities with the good unless they are directly and expressly the controlling point for the transitions of both cash->good->cash. Otherwise a whole bunch of standard economic systems can be considered gambling.
As for "maybe the world would be better if children couldn't be manipulated into spending all their money on packs searching for rare cards they have no chance to get". Maybe the world would be a better place if parents actually raised and taught their kids and the kids can make rational, weighted decisions about whether the chance to get a card they want is worth it or not.