I always wondered this myself.

I think I remember reading a while ago that apps need to be updated occasionally to comply with .apk guidelines.

This is presumably dependant on which permissions the .apk requests. So a simple calculator app wouldn’t need to be updated, but the calculator plus app, which tracks my blood pressure, how many glasses of water I’ve drank, and my semen count, would need to be updated occasionally to comply with Google’s privacy policies.