I guess because the implementations aren't intending to support a lot of different input codecs, so there's no benefits to the flexibility of AIFF/AIFC. Also avoids having to deal with an 80-bit extended precision float just for the sample rate.

Hijacking this comment in case you (or someone from HN) knows.

Is there any particular reason that AIFF uses an 80-bit extended float precision for the sample rate, instead of a 32-bit integer like WAVE?

I assume this was an oversight at the time, but I'm curious if there are actual applications where sub-Hz (or 2^32Hz) precision is needed.