Gambling is a basic part of the game: you need to buy keys and they open a box with random contents. If I open a box with a Dragon Lore AWP skin, I may be able to sell it on the community market for $800 dollar, which after paying a commission to Valve is deposited straight in my Steam account by Valve, which I can then turn into $800 dollars in my paypal account by visiting a site they support. It's gambling for real money and they know they're dealing with kids doing the gambling.
I think you're mistaking the meaning of "basic".
You can absolutely play CSGO for years, try all of it's game modes and NEVER have to use a damn skin or open a box. In fact lots of people do.
Just because a minority of players decide to waste ridiculous amounts of time and money in a side aspect of the game doesn't make that aspect "basic" at all.
Also, please explain to me how are pokemon and many other card games not forbidden to kids, since to you they fill the definition of gambling.
P.S.: 99% of dragon lores don't come from boxes, they drop randomly for operation players without need to buy any keys, get your damn facts straight.