it is so rare to see an under 18yrs taking alcohol
It is because that you will not see minors doing it in the public streets. They're doing it in closed door areas where no one will notice them or else they'll get arrested for that.
but in gambling an under 18yrs can gamble without being noticed even though it is strictly for 18yrs and above. now, To answer your question, alcohol has been around for so long and that is why it is more accepted than gambling.
It's just the same for both.
They've been there since ages and as well as can be done where it cannot be noticed that the underage kids have been doing it.