Why? It is natural that game companies gambling through microtransactions is a fundamentally bad thing, but I would say that gambling with ingame funds is entirely fine, since it teaches you the consequences of losing.
A teen never lives with the immediate consequences of one's wealth when a credit/debit card is tapped in a transaction. The pain comes later when the bills come in. However, losing 60% of your ingame gold in one poor bet, has immediate consequences in the sense that you will clearly have less to gamble with the next time. Maybe you think "I can recover, I can do it, I just need to gamble my last 40% and I can win!", and you lose it, ending up broke. Well, back to earning money again, or you get so broken up that you quit the game.
Either way, you learn a valuable lesson. Stupidity has consequences, and perhaps you should not gamble if you cannot handle the consequences.
But for those players that are rich, bored and belong to the 1%, gambling is one of the few thrills left, as I assume PvP is still in shambles.