why do gamblers tend to wait until they have suffered a catastrophic loss or a string of losses before quitting or seeking help?
Because when one takes some initial losses, one does not stop and try to outsmart the casino, chasing those losses.
Also, loss chasing is not something of a single gambling session, there are people who actually keep a record of all the money they have lost to the casino and return to gamble regularly in hopes of turning those numbers to their favor.
They mostly talk about it when they realize there is a problem or when they hit the bottom, otherwise, they still have a miracle recovery in their mind.