Because otherwise someone will spend a crap Christmas alone (when they don't want to and have made it clear they don't want to).
Surely, just as we expect men to be an adult and help and do the mental load, we can expect people to be adults and sort their own fucking plans?
If you do t want to spend a day alone, make some fucking plans
Absolutely, if the person is not elderly or frail in some way, or in poor mental health, or if their husband hasn't killed himself, or is recently widowed (the latter two apply to two separate individuals in my family for example).
Call me old-fashioned, but sometimes it is simply the right thing to do although I know that view isn't popular and leaves me wide open to accusations of martyrdom.
It's not that you actively hate having them to stay, it's just that you may not choose to at that particular moment, owing to other stuff going on in your life, but Christmas is a fixed point.
All I am saying that this isn't simply a black and white situation and it's sometimes not as easy as saying "right, the drawbridge is going up this year". It may be possible for a couple of years here and there to do that, but not every year.