In television, film, and especially comics, it is conventional to focus on the upper body, since the upper body is usually the most expressive part of any character. Face, arms, and torso all convey more information to the audience than the legs. In some works, you're doing little more than watching actors talk, and anything other than the face is superfluous anyway. "Not Wearing Pants" Dream is where this happens in a dream sequence, but this is when the audience (at least initially) cannot see that the character is pantsless, whether it's in a dream or not.