It is not considered as parasite because sporozoites first enter by bite of a female anopheles mosquito and then it produces gametes in human host and when another female anopheles mosquito bites then gametes are transferred to it and sexual stages develop in mosquito only and fertilisation and development also takes place inside mosquito.

they aren't considered parasite because they don't on the host even for short duration mosquitoes sucks blood and leave unlike parasites which remain there for sometime.