The reason that hasn't happened is because `#!` is a kernel level thing, but the `PATH` env is a userspace thing. I suspect even adding env parsing to the kernel would not be viewed as a favorable change…
It could literally be a shorthand for /usr/bin/env, actually delegating the PATH handling to user space. The path to /usr/bin/env could be a boot arg or runtime variable (sysctl). It could probably be implemented in Linux (without touching kernel code) via binfmt_misc. It's mostly a matter of people agreeing to do this.
