Its bikeshedding - they can see it so they have an opinion on it. I think it happens in many fields where the output is visual - photography, advertising,.....

There is also a general feeling that websites are primarily about design (rather than development) and that the design is aesthetic (rather than UI).

> I can't imagine another field—hairdressing, construction, financial advice

For financial advice, maybe not as readily, but it definitely happens pretty firmly. Lots of people have lost money taking risks they have been warned about. A lot during booms because of FOMO, and a lot because people do not even take advice in the first place.