I think this is how it has been since the beginning of ELF in Linux. PT_INTERP comes from the original TIS specification of ELF and I think it was probably also in the original SVR4 ELF implementation.

I understand why they went this route. While it is unfortunate to need duplicate code parsing and loading ELF files, the ELF binfmt in the kernel is at least relatively simple, since it does not need to worry about dynamic linking. Doing what QNX did would be possible, but it would also add moving parts and change the relationship Linux has with the userland, which is one thing they do not like to do. They could probably come up with a middleground, like pre-baking a raw memory image with an ELF loader that can be stuck into a new process when exec'ing an ELF binary and shipping that with the kernel, but I'm sure there would be observable side-effects with regards to performance and maybe locks, I can see it being more impactful to focus on ensuring the existing implementation is correct. (AFAIK it is still "only" a few thousand lines.)