One thing nobody has ever explained adequately to me is why you DON'T want to transpose a song.

Wow, Joanne, I have never heard that...Do you recall who said it, or where you heard it? I don't think I've led a musically sheltered life. I've been singing since I first grew teeth, strumming at least since age 7, then piano lessons, spent 30+ years adulthood messing about with any instrument I might get my hands on to see if I could get it to go. Whether at home, campfires, coffee houses, song circles, choirs, handbell festivals, barbershop competitions...transposition is everyday life. Is the idea that transposing is bad possibly a tacit tune session courtesy, to be thoughtful of those who don't have that particular skill?

There are two other situations where I can imagine it wouldn't be a popular option: 1. Classical music--never saw it done except for beginners. People read their music, which is often complex and written to be do-able by certain instruments in certain keys. And pre-computer era it was a LOT of labour to transpose, for example, an orchestra's worth of parts plus conductor's score. So it's in the culture not to do it. Might evolve with time, might not.
2. For people with perfect pitch: The few I know who have it are bothered by hearing music in a new key once they've become familiar with it in one key; the new one sounds out of tune to them.