Because it's delicious, that's why! (As long as you add the requisite mountain of shredded cheese, of course.)
True story: As a child, my cousin and I planned and performed an elaborate dance to "I Want a Hippopotamus for Christmas" for our family. We still get asked to do it every year.
I was at Fenwick's the other day and actually heard traditional Christmas carols instead of the crappy pop songs that seem to be the only thing Brits listen to. I was in heaven!