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!)
Maybe that adapter gets so popular that a phone designer comes up with the revolutionary idea of integrating it into the phone itself.
Those who were around during the days of early 'home computers' - think Commodore bread boxes, Sinclair door stops etc - might remember the evolution from those devices which generally lived in a jungle of cords emenating from their sides and backs, connecting them to power supplies and peripherals to the more streamlined versions which came later - Commodore 128D, Amstrad all-in-ones, various species of MSX and yes, the original Apple Macintosh - which had most of the essentials built-in. Having one of those on your desk really separated you from the plebs who still lived in the jungle.
Apple seems to have forgotten that part of its heritage.
IIRC you can't charge at the same time as using the included one, and need to pay extra for the bulkier one that allows charging and use of a 3.5mm jack at the same time.
That is OK much of the time, but a faf is you want use your iDevice for music while working for some hours or while mobile for long enough that you might normally plug in a portable battery.
Unless they've started including the two function adaptor as standard but that will irritate people who don't want the extra bulk...
They do not include that one as standard, as far as I know.
If you are someone who charges your phone while listening to music on a regular basis, then I can see the lack of headphone jack being an issue.
The real solution for that problem, in my opinion, is to just put two USB C or two lightning ports on the phone. They have much more usage than a 3.5mm port. Two places to charge from, charge and have external storage attached, etc.
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!)