My car is old and I can't afford a new one, it's 3.5mm or a bluetooth speaker in the passenger seat.

Because the extra circuits will increase the bill of materials by a few dollars and there's not enough margin for that

I don't carry a bag and I don't want to put a dongle in my pocket

Currently I can connect my phone to a set of speakers at a small local café, several of my expensive over ear headphones, aux input of my friend's car, and a 70 cent lapel mic. Can I carry a dongle with me everywhere? Probably. Will I remember to do it? Probably not.

One note I noticed about USB-C dongles and headphones is that they need to be powered, whereas traditional headphones do not. So the headphones draw power from the phone, which then hurts battery life.

A lot of the answers are good and make sense. Sometimes folks have an impression that wired is better even on some super old, crappy headphones.

If they love those wired buds, they won't want a phone without the jack.

Source: My fiance's mom thinks her new phone is bad since it doesn't work with her ancient crappy earbuds.

Great answers here that I 100% agree with, can't add much

Because all USB C to 3.5mm Aux adapters are flimsy as mother fuckers that break down after two months use. I would not even care otherwise, I never charge and listen at same time anyway.

If anyone has suggestions for adapter that is not made out of thinnest possible wire and is durable, let me know.

Also, I don't want to buy USB-C headphones, since I would only use those with my phone, I want to use them also on other devices, and for compatibility it is better to have it analog instead of USB-C. If I were to buy headphones for phone only, I would just get wireless.

I often have use cases at work where I have to plug in my headphones to device I am not familiar with, for audio troubleshooting at our customers device. Most of the times USB is not an option, only standard analog audio.

Modern laptops also come with way too few USB ports, 2.0 and C combined, so I rather not waste one for audio since there is no reason for audio to go through USB. On my main PC I don't use my internal soundcard but external audio interface for music production, and I want my headphones connected to that, not USB.

So in conclusion, USB-C headphones would be totally worthless to me, no matter the device. Even for phone, I rather go with adapter, or just wireless ones.