Not arguing with you btw, but the excuses they've provided are all BS. For starters, IP rating isn't really an issue - the Pixel 5a, Galaxy A52, Xperia 5 V, Zenfone 10 - all have a headphone jack, and a minimum of IP65, even going up to IP68 (Pixel 5a), so that really can't be an excuse. Also, cost can't be an excuse either, considering the examples I've provided consists of both budget and premium phones.
The only answer that makes sense here is GREED. Nothing but plain and good ol capitalism at work.