Sometimes this is because they are at an earlier stage in their career, sometimes because their career stalled after they were discovered hiding in the chimney of the dean's apartment. These people can be fired, but if they are, it will not be for reasons related to their ability to teach calculus. In Europe, Assistant Professors really are assistants, whose job is to mow the Professor's lawn, carry their briefcase, and teach their class. In the U.S., the Assistant title just means that they are in the preliminary, untenured stage of their career. It does not necessarily mean that they have anywhere to go to afterwards.
Some colleges hire temporary faculty mainly to teach classes.
This may mean that they really care about their teaching. It may mean that the instructor is too disorganized to post a sign or that revengeful former students keep ripping it down. Could mean that the instructor is deemed unworthy of an office. This makes it hard to hold office hours. Also could mean that you're looking in the wrong building.