Boaz poses a great question! Why are Plankton not insects, when they look very similar? It all has to do with taxonomy. There are many species of plankton, just as there are insects, and some of them are not even in the Animal Kingdom, as insects are. Of those species that do belong to the Animal Kingdom, they fall in Class Crustacea, and are related more to lobsters or shrimp than insects.
To see the wide variation of Kingdoms, Phylums, and Classes in which Plankton fall, check out this website.