In Polish, earthworms are "dżdżownica" which is derived from old Slavic for rain.

Yet again, I'm jealous of the potential Scrabble scores Polish has over English

In Swedish they are "daggmask" which translates literally as "dew-worm", so the moisture connection is clearly there, too. Fun!

Their behaviour to surface during rain and get maximum human attention likely to get them named after "rain" across cultures.