Because interpreting JS without JIT would drain the battery and make iOS look bad to its users who would not understand that the poor battery time was due to the third-party browser they are using.
Nope, third party browsers used to be unable to use JIT even when using WebKit, and that was back when devices were slower. This meant there were jailbreak tweaks such as this one: http://cydia.saurik.com/package/net.joedj.nitrous/
> Nitro dramatically improves JavaScript performance in web-oriented applications like Chrome/Facebook/Twitter /AlienBlue/Dolphin/Mercury, typically making it 4x to 5x faster.
They changed this in iOS 8, but not many people really noticed even with the slower devices from back then (at least I don't think they noticed that it had been slower)
For every improvement that gets added to browsers, in executing JS faster, web developers will take that extra perf and spend it to do more. The result of that is that only the heavily optimised browsers will be able to deliver the experience that the people expect.
A browser without JIT today, would be a no-go. It would be noticeably slower, and it would drain the battery faster.
https://v8.dev/blog/jitless says that while some benckmarks can be 80% slower without a JIT, real world performance in the YouTube web app is more like 6% slower. To be fair that probably doesn't rely on that much JS, but it would be totally feasible to use a browser without a JIT even with modern web apps. Edge has a secure mode that disables the JIT.
Of course, the proper solution would be to allow 3rd party apps to use JIT, but allowing 3rd party engines without JIT would definitely be better than nothing
