As many have said, you're paying for chefs, potwash, waiting staff and bar staff to serve you on Christmas Day, that's why it's expensive. They are spending a national holiday that's traditionally spent with family, giving other people the opportunity to not shop, plan, buy, cook and clear up, while spending time with their family. Interesting that some people don't factor that in at all and yet will expect happy staff that give them a great experience, but clearly don't want to pay for it.
It's also an absolute ball ache to organise admin wise and then shifting tables around, setting up , all the prep etc - it's not just the people on the day, it's the people who come before, setting the room and then clearing it all up after, preparing all the food etc.
And when doing it you're very aware that people want things to be 'perfect' and at least where I've worked a lot of time goes into the planning of that. And every single table will have a different idea of perfect.
And hospitality is struggling for staff, especially chefs, you want them to work Christmas Day so you can spend it being waited on then it's going to cost you.
But sadly at the moment society seems really down on hospitality and the staff for some reason, so I'm not surprised.