"Por quê nao poderia ser "I hope it not rain" ou algo parecido? No segundo caso, a ação ainda não aconteceu, assim como no primeiro, então por quê usamos o simple present?"

I hope you inform us the sources of your "pesquisas"

BTW, "I hope it doesn't rain" is grammatically correct.

Eu espero que não chova - subjuntivo "chova"

I hope it doesn't rain

Eu espero [ que ] não "chove"

In Portuguese the subjunctive of the verb chover is chova, que chova, que não chova but in English, and the focus here is on the verb rain and not on the verb hope, we use the 3rd person singular for the verb "to rain".

It doesn't matter if the sentence in Portuguese is in the subjunctive because the game here is not translating but communicating in English in a proper manner so that native speakers can understand easily what we are saying or telling them.