Why would smart people care about denim vs. trousers?
Let competitors wear pajamas- it makes no difference.
More generally, appearances are important because they are clear signs of attention and care. Something worth our respect is worth dressing up for, and a collective dressing up reinforces the importance and elevation of a given event or moment over other events or moments of lesser import.
4.10.1.3. No players with t-shirts, jeans, shorts,
sneakers, baseball caps or inappropriate dress are allowed
in the playing area. Any requests to wear national or
traditional dress shall be approved by FIDE Supervisor.
Reference: https://www.fide.com/docs/regulations/wrbc_regulations_2024_...