the truth is that people love to gamble rather than invest. Maybe they'd just know about Bitcoin because it was already been an option now in gambling sites but thinking this would give them interest, I don't think so. Because as long as fiat still exists, we can't assure that gamblers will choose Bitcoin, with the high fees and transaction delay - that was a big deal for them.
Yes, we can't compare Bitcoin to gambling in any aspect and much more about popularity because whether we like it or not, Gambling has been known for many years and people are happy despite suffering losses.
You are right when you say that and even investing itself can also be considered as gambling because we are staking our money in a volatile asset which has an unpredictable future and whose price is subject to a lot of factors. Before crypto gambling exist, crypto came out first so people already know Bitcoin just before they step in, inside a crypto casino. Some crypto casinos has an invest in bankroll feature and staking and both of them can give us an interest. Fiat is different from Bitcoin, so there will always be people who choose Bitcoin over it or they will choose to play on a crypto casino because it can provide them anonymity.