True. It's about specialization. You could also ask if programmers make good database administrators. Highly capable/experienced programmers that develop for the Web (like many on HN) will have some level of knowledge of all facets of the network computing experience, including how routers and browsers work. However, their primary focus and highest area of knowledge should be programming, hence their title.

An analogy might be basketball. Growing up you learn to just get the ball in the hoop, but those that turn professional specialize in a certain position. Very few can play all five positions quite well (like Magic Johnson).