Well that is because our hands have usually touched the fish & we have oils in/on our pores.

My dad often told me of his travels during the Great Depression where he "first" witnessed "raining fish" & "raining frogs". He never joked so I knew it to be true. He & others would gather up the 4" and bigger "poddies"(fish: no frogs) to make meals. I was amazed they could still be jumping around after such a fall & he assured me they were still very much alive after hitting the ground(albeit somewhat wet).

We have all heard of "raining cats & dogs" from our Brittish/European ancestory where animals were located in the ceiling during heavy wets. Unfortunately there wasn't a propper floor above & animals would (( probably (I think) from scuffling for room)) fall through onto the people below.