Well, that’s a software limitation and, of course, there is an app for that: Halide.
In a Reddit post, Halide developers have posted information that the human-limit in the iPhone XR’s portrait mode can be circumvented using software and the same edge detection technology may be used for people.
In fact, they got the functionality up and running in an internal test release, as Halide showed on Twitter. Here is a demonstration, comparing the XS and the XR on the same cat.
According to the Halide developers, the feature is a bit temperamental, but it's capable of capturing depth maps on everything from cats to soda cans.
It seems that soon, dogs everywhere will agree that the iPhone XR is indeed the best phone for the money. Cats still hate humans taking their photo, so they will likely continue to disagree.