몹시 means very so it usually has to modify an adjective/adverb. like 몹시 피곤해요 - very tired 오다 is a verb meaning to come, so when people say 비가 와요 they're saying rain is coming/rain comes. Just like in English, it's a bit strange to say "It's very raining", "something very comes, something is very coming". If you want to say "It's raining a lot" people typically say "비가 많이 와요."