> I don't see why the developer that builds the interaction with the backend can't be given directly the design and turn it into HTML and CSS himself.

Because well-engineered HTML and CSS that works across all major browsers and can handle everything from tiny phones to high res desktop sizes and meets accessibility requirements is a very difficult skill to acquire. In addition, that skill has very little to do with the skills required in more traditional programming languages, even JavaScript.

It's perfectly possible to have someone fulfill both roles (I do, but I'm the senior-most FE person at my company.) It just takes a long time to master both sides of that equation.

Everyone should be able to do basic HTML & CSS, but deep expertise as I described above requires either a specialist or someone with a lot of experience.