It is not only an issue of pay (although that is part of it) but it is also an issue of taxes and just plain ole hard work.
It is kinda like the same reason folks that live in America won't do the jobs that those who come to America (legally or otherwise) are willing to do.
A Social Host makes 'about' 1,500.00 a month last time I checked. Now that is not great pay, but considering you don't have any expenses (other than personal items) it is not bad for a young person who wants to see the world.
The cruise line pays for your room, board, health insurance.
So for an American...if a young person lives with Mom and Dad back home and has no car note... then they can see the world and make a little money.
For folks from other countries, this would be considered GREAT money.