The most of the gamblers are so addictive, so it's so hard for them to stop gambling even if they lose a lot of money they still continue which is totally so bad, it's really a disease.
The op is asking for the valid reason, and the reason simply is addiction! I know a lot of players that we're not addicted an at first, but after a period they start to be addicted to gambling and they can't stop it. I think the reason here is the greed. Anyway, they always end up financially broken.