Here's why:
* "Rains" is the third person singular present tense form of the verb "to rain."
* "Rain" is the base form of the verb, which is used for all other tenses except the third person singular present tense.
Here are some examples:
* It rains every day in the rainforest. (third person singular present tense)
* It rained yesterday. (past tense)
* It will rain tomorrow. (future tense)
* Let it rain! (imperative)