You do say 'it rains', don't you?
These verbs are called defective verbs. I can think of 2 categories of Spanish defective verbs: those which can only be used in 3rd person (such as llover, nevar, etc.) and those which lack specific tenses (such as abolir [abolish]). Saying Yo abolo la ley is absolutely incorrect, for instance.
A "defective verb" is one which has an incomplete conjugation. "To rain" is defective in normal usage, but it can be used in other persons, so it is not actually defective.
"It rains" is passive voice. One can use "to rain" in active voice, where it is a transitive verb. "The large airplanes rain down thousands of bombs upon the defenseless city." "If you do not obey me, " said the Queen, "I will rain down upon you the torments of heaven." "God rained down Manna upon the Israelites in the desert."
This is one instance where Duo's literal translation would lead a non-native speaker to adopt a idiom which is not good speaking English. After decades of interacting with English speakers, it can only conclude that the "correct" translation of both llueve and no llueve is "It's/it is rainoing" and "It's not/it is not raining." People just don't say, "it does not rain", they just don't.