Well, if you look at the specs for that iPhone XS Max that I referenced in my original post, it’s screen is about the same resolution as a 13" MacBook Pro, it has as much storage as a mid-range MacBook Pro, and if last years’ benchmarks are anything to go by, it’s processor will probably be competitive with at least the entry-level MacBook Pro. Considering what you get, it shouldn’t really be surprising that it costs as much as a MacBook Pro.
Now, one could argue that you don’t really need a MacBook Pro in your pocket, and if you come to that conclusion Apple offers a 128gb iPhone XR for $799.
I’ve used mine twice (on two different phones), both times for a cracked screen.
Frankly, for me the biggest part of its value is giving me the confidence to go caseless on my iPhones.