Because it can be a really special, magical belief to have and for the family to indulge in. Which is a lovely thing that the DC and parents will always look back on fondly.
But this is MN where you'll always get posters sneering about it and saying it's not the only way for Xmas to be special or magical, despite no-one saying it was.
And for all the millions of people who say it was a lovely part of their childhood that wasn't harmful in any way, there's always a poster or 2 who'll claim that finding out the truth made them realise their parents were liars and their childhood was shattered and they haven't trusted anyone since or some other nonsense.
Just do you.
It's like f-ing clockwork on MN this time of year, you get the Santa questions early then the 'Halloween wasn't even a thing when I was growing up, it's American' and all the complaints about Halloween and how DC shouldn't be trick or treating or if they are, should be given raisins and cherry tomatoes instead of sweets.
Then the complaints about people putting their Xmas decorations up too early. And the complaints about fireworks night being antisocial.
Then complaints about advent calendars because chocolate ones containing 2g of chocolate a day contain too much sugar but little gifts each day or non-chocolate advent calendars are too many presents and spoiling DC.
And North Pole breakfasts, December or Xmas Eve boxes are just too ridiculous, common, tacky, performative, and lead to spoiled DC.
Then all the other arguments about how many gifts DC should get and what should be in a Xmas stocking as otherwise it's just extra presents and not a stocking if it isn't an orange, a toothbrush, school socks or whatever.
Then all the arguments about whether adults should get or expect presents. About whether family at Xmas means you, DP and kids or other people. How much is acceptable for DCs to get at Xmas with some spending hundreds and others doing the 4 presents max 'something they want, something they need, something to wear and something to read' trope and railing about not having plastic tat in their house and all their kids get 2nd hand wooden, educational or biodegradable toys. And the "just wrap up an empty box, they'll enjoy that just as much as a toy" advice for any DC under 2 and even wanting to buy a under 2 year old a present is pointless, excessive and a bit common.
Then 26th of December the complaints start about people taking their decorations down 'too early".
Same every year.