@VestaTilley
Because, frankly, they’re morons.
As religious literacy in this country declines, so too does the understanding of what Christmas is, what Advent is, and when both begin.
The same people will have it all down by Boxing Day, thereby spectacularly missing the point.
It is now generally accepted that Jesus was born in the autumn.
Shepherds were 'tending their flocks in the field' - the latest they would be out is October or very early November.
When a census was called, it was never in the winter because travel was too difficult.
A previously unknown slow moving comet appeared in the sky at about that time, and is believed to be the Star of Bethlehem. The Kings weren't kings at all, they were probably astronomers who believed that this was so significant (it was very bright and visible for many days - science can count back) that it had to be meaningful. The best understanding of the comet also suggests autumn.
Correlating Jesus' birth with other biblical events which can be dated also suggest that it was autumn. However not all scholars agree on this. Some argue (again giving reasons) that it was in the spring, and one or two that it was in June.
What is certain is that whenever it was, it was a day when we are most likely at work, or pushing a trolley round Tesco, or we're cleaning the loo. Not the day we're stuffing ourselves silly and trying to work out how to see all the parents and in laws without offending somebody. And I confess to being irritated watching Midnight Mass on tv and hearing the celebrant saying 'on this day Christ was born'. No he wasn't. And you know he wasn't. You know that many centuries ago the feast of Saturnalia was repurposed to represent the day Christ was born. So by all means say at Mass that this is the day we celebrate his birth, but don't tell us this is the day he was born.