Rick, for the same reason you might tip or present a gift to any of the people who provide personal services to you on land. Some people (but by no means all) tip their newspaper carrier, their mail carrier and any other service provides during the holidays. I usually bring a nice bottle of wine to my barber when I get my pre-holiday haircut. Why? Well, because I want to and, thank God, I can. My way of saying thanks for the great service during the past twelve months.
I can't even begin to imagine the holidays being thousands of miles away from my extended family, but that's the situation cruise staff find themselves in and they're surrounded by a couple of thousand people who are celebrating a holiday. As a tip or a gift is always voluntary, it's entirely up to the individual giver as to whether or not they want to give something, but I think it's a kind thing to do.
(And my friends think I'm a curmudgeon!)