More stereos and cars don't have BT than do (significantly many more in my experience). Bluetooth pairing is a faf compared to just plugging in a jack, and many devices still only remember pairing with two devices or just one so your friend may have to repair when you are done. Double adaptors (I still see people sharing a single audio source on headphones that way) are a thing with simple audio jack that there isn't a BT solution for (at least when I've tried phones won't play down two BT audio sinks at the same time).
I've used the audio jack on my phones only a couple of times in the last year or so as I do use BT for general headphone use, but that is a couple of times more than zero and in all those cases BT would not have been an option.
Of course the obvious answer is: if you want a jack and Apple doesn't provide one on their devices then switch allegiance - but a lot of people feel tied to Apple for other reasons (lock in through apps they'd have to repurchase on Android, lock-in through add-on device compatibility, or just the extra usability that comes from personal or group familiarity) and would like to stay without losing that feature.
Another option is bluetooth receivers, which can be bought quite cheaply these days: pair one of those with your iDevice and plug other devices into that when needed. That is an extra device (a very small one, but still an extra) to carry around and remember to keep charged and I doubt it will happen much because people won't even carry charging adaptors around with them (the amount of times I've have the look that suggests it is my fault that someone can't borrow my portable battery because their device doesn't follow the same standard as everything else and they didn't bring an adaptor!)