The question "在班上有60个同学，为什么我只看到59个?" is a riddle. The answer could be a trick — perhaps one student is absent, or, more likely, the speaker is part of the 60 but not counting themselves. Many Chinese puzzles like this have answers that play on the speaker’s perspective.
In this case, it seems like the answer might be "因为我没有看到自己" (Because I don't count myself).
