Why doesn't rain on the superstructure at an electric substation conduct electricity back to the frame?

Why doesn't rain on the superstructure at an electric substation conduct electricity back to the frame?
Who says it does not?
Go there on a rainy day and see for yourself : frequent sparking does occur.
Note that pure water is a bad conductor. Water needs to be polluted to conduct electricity.
For instance with salt added to the water it becomes an excellent conductor.
Rain is always polluted by microscopic `seeds´ of stone or sand, that allows water vapor to collect and build up around the `seed´. Depending on what `seed´ was used to form the raindrops, conduction is worse or better.

Piped water is on the contrary loaded with salts and other pollution. Making it often a better conductor than rain (with exception of `acid rain´).
;)

If you could ever get to see one of the substations up close , you would see so many ground circuits it would boggle your mind. THat is also why lightening is not a great concern to these facilities, They are VERY WELL grounded to delute the effects of lightening and arcing.