Mostly it's about the sponsors. It's much more difficult to get sponsors for an event if the participants are dressed like they slept in their clothes. That's why organizers try to impose minimal standards on dresscodes.
Jeans and sneakers are maybe debatable, but players showed up with cargo pants, shorts or tank tops on other events.
In the FIDE regulation for that event jeans were explicitly mentioned as not allowed. FIDE would have made a fool out of themselves when allowing Magnus to wear the jeans.
Would be interesting if they can get mattress companies or apparel companies that have good comfy clothes as sponsors. Why not play chess on a firm mattress?
FIDE needs to embrace the younger generation that think the game is cool. Ancient dress codes are a distraction.
Textbook slippery slope fallacy.
Also, he looked very sharp in his outfit with the jeans. Frankly it was a better ensemble than I’d look in one of my suits.
This looks to me like a case where FIDE got greedy and forgot to balance the talents interests with the sponsors.
Anyone who considers jeans to look like "clothes someone would sleep in" is immediately dubious in my book. Jeans are so extraordinarily uncomfortable to sleep in that I don't think I've ever intentionally done that in my life.