Lots of people do. On my first Celebrity cruise I met a couple that lived on the ship--the husband owned his own business and worked remotely during the day while the wife home-schooled their two elementary school-aged kids, then during the evening the husband played $3,000 per hand Blackjack (I think--it was whatever the table maximum was). They were comp'd 2-bedroom Penthouse Suites (or better) on every sailing. That's an extreme, but for ships that have casinos it's pretty rare that I'm on a semi-premium cruise line and don't see at least a few people playing $150+ per hand blackjack and $25+ per spin slots. But for many people, being up or down $5,000 at a time isn't really all that much money anyway. Aside from the occasional whale the casinos tend to be pretty full during the evenings (probably at least partially because a lot of cruise lines have dialed back on evening entertainment options).
Even so, I think DISers tend to overestimate the impact that the casino has on ship revenue. It's meaningful, but it's nowhere enough to justify the Disney price difference. (Not to mention that Disney has bingo, which is probably the biggest gambling rip-off of them all.)