Usb is a carrying digital signals for the most part while headphonea are analog. So you have 2 options:
Implement some janky non-standard analog audio in some usb alternate mode that will only work with some devices
Add a digital to analog converter in the plug itself, which is more complex and expensive
Both options prevent you from chanrging while using headphones unless you use a splitter or wireless charging which is not ideal
3.5 mm may not be the king, but it’s extremly simple and fairly robust and has little to no disatvantages except for wireless headphones sales ofc. You don’t have to charge these heqdphones, you don’t have to worry about losing one of them.
Wireless headphonwa have their uses, I doubt they benefit the majority of people tho.