One reason its cheap ...
Long hours and low pay - Some workers face shifts of more than 12 hours a day, seven days a week, often without overtime pay. Wages can be very low, sometimes below $20 per day, though tips can supplement income. Workers often live in small, shared cabins with limited personal space. Ships often registered in countries with lax regulations. No pay between workers contracts
These are ONLY some of the reasons ....
It helps to be on a Panamanian registered vessel in international waters.
Go price out hotels and food in any major destination for one week. Now go price out a cruise for one week which also includes entertainment and a travel component. Somehow, the cruise is CHEAPER and offers more.
That's it. That's the whole answer.