A UNIX-style process spawning interface, like exec() or posix_spawn(), requires the kernel to have some awareness of the executable format so that it can create a process from an executable image. It doesn't have to support every feature of the format, though.

A model in which this happens in userspace is conceivable, but it'd be pretty weird. Besides, it introduces a chicken-and-egg problem -- if you need a process to launch a process, you need some sort of special case for init.