why's it considered rude to tell a caller on the phone that the person they're looking for is in the shower?

Because the person on the other end of the phone probably doesn't want to imagine that person in the shower, especially if it's someone they don't know very well. I usually just say that they're "unavailable" or "unable to come to the phone right now," and ask if they want me to take a message.

I don't think it's rude. I agree with Bella, people shower/bathe everyday or they should.

Besides it's better than saying, uh...he's in the bathroom, can he call ya back?

its not as long as

you ask the caller

can you take a message

maybe its just me but i dont think its rude so what people shower everyone does it since when is showering such a bad thing lol

I think we expect that as a person grows up they become capable of reserving certain details and just saying they aren't available.

Because washing yourself in the nude is still considered somewhat personal, I would assume. You should say they are "indisposed."

never thought about it, but i suppose so.

Just say "he/she is unavailable, can i take a message? I'll tell him/her to call you back"

You should just say that they are unavailable.