OP wrote:
"So, seriously. Why are are all buying into this 20 people round a table, with a Hogwarts spread, bullshit?"
We are not all buying into this.
We are a small family unit with no young children and no close family left alive. We have chosen not to do anything much about Christmas for years, now. It suits all of us.
You do not have to "buy into" the lavish festivities portrayed in Christmas adverts and the media.
I see dispiriting threads on MN where the OP lists how many presents they have bought or how many Christmas activities they have already done with their kids, asking - "Have I done enough?"
"Enough" according to whose criteria and according to whose pocket?
As kids, we had simple Christmases; trees and decorations did not go up until a few days before Christmas Day. There were carol concerts, school and church events and maybe a panto or ballet.
Now it often seems a depressing contest that kicks off on 1 December to see who can fit in the most "Christmas memories" - plus the Polar breakfasts, the Christmas Eve boxes, the matching PJs for the entire family, the Christmas bedding, the "Reindeer experience"...
You do not have to buy into any of this.
Who gains from this frenzy of competitive and expensive "memory making"?
John Lewis, Waitrose, Tesco, Sainsburys, Amazon et al.
As NibbledSwitch has said:
"It's all fucking madness!"
OP wrote:
"We celebrate it blindly, and don’t even know why we are doing things we do."
It's a tad patronising to assume that all of us don't know why we are "doing things". There are people on MN who will be old enough to recall what Christmases were like in the 50s, 60s and 70s.