Most of the alienss in television and movie science fiction shows are humanoid, since it is easier for an interactive character to be a disguised human actor. However, there are various methods for presenting non-humanoid characters, for example computer graphics, creative costuming, and puppets.
Some fans find that it rings false to have a universe populated by creatures that look human. Occasionally, shows present a reason for this to be the case. For example, the episode "The Chase" of Star Trek: The Next Generation explained the humanoid denizens of the Star Trek universe by advancing the story of a primordial humanoid civilization that seeded the galaxy with their DNA, thus causing other humanoid species to emerge.
In most cases, however, the reason for the similarity is not explained, and it is regarded simply as a dramatic convention.