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.

The expertise offered here is "how to build a website". If the client is insisting that the dev use a specific javascript library, that would be odd.

The client here is just requesting specific content on their website, similar to someone requesting a granite countertop in their kitchen; that seems fine, even if its not particularly classy or aesthetically pleasing to the contractor.

But honestly, they are more likely to NOT be experts in the business of the client. They are experts on tech, their own business and aesthetic.

People come to hairdressers with own ideas about how their hair should look like and reject hairdressers advice. In fact, hairdressers are not even trying to give you advice unless you explicitly ask for it. They sometimes makes mild suggestions and offers, but that is it.

Frankly, financial advisors are more likely to give advice designed to max out their bonuses rather then one good for you. You probably should firmly reject that financial product or flat tire insurance.