> It would take maybe 15 lines of code to implement

This is a very bad reason to justify something. Especially introduce threads. Your response here is like saying "I don't know why people say it's so hard to write multi threaded programs, the thread create API is so simple." It completely misses the point why added complexity can be harmful.

> without rarely occurring bugs.

Except for a glaring thing like "what if fflush gets an I/O error in this background thread"?

> Granted, it would rub me the wrong way if libc did this by default,

This is exactly my point. It needs cooperation from the application layer. It wouldn't make sense to be transparent.

Almost no one checks for error when printing to stdout. That's why SIGPIPE exists.

Complexity, readability, etc. is the argument people make when they've run out of arguments.