Personally, I think it's because many clients are used to years of hearing "programmer" and "web designer" as two separate jobs. If you're writing desktop apps, well, you're a programmer. If you're doing web-based stuff, you must be "just" a web designer.
And yes, you can easily do non-trivial stuff with a little light HTML and Javascript, but once you start getting into serious CSS, PHP, etc. it ceases to be something that anyone can pick up in an afternoon. In essence it becomes the same sort of professional work that "programmers" do.
But to the client there's still "programmers" and "web designers", with the prestige and the dollars definitely being associated (in their minds, anyway) with the former.
And yes, you can easily do non-trivial stuff with a little light HTML and Javascript, but once you start getting into serious CSS, PHP, etc. it ceases to be something that anyone can pick up in an afternoon. In essence it becomes the same sort of professional work that "programmers" do.
But to the client there's still "programmers" and "web designers", with the prestige and the dollars definitely being associated (in their minds, anyway) with the former.