Today the kernel does create the swap file itself. I don't know why it behaved differently in the past, given that the version of dynamic_pager I linked is only 355 lines of code, not obviously complex enough to be worth offloading to userspace. But this was written back in 1999 and maybe there was more enthusiasm for being microkernel-y (even if they had already backed away from full Mach paging).
Looking at some of the contemporary documentation it does look like it was essentially a historical accident. The interface it built to be all microkernel, but when it was adapted into a real system the microkernel concepts fell to the wayside as they were no longer useful, but where they didn't impose too much of a burden (as in the pager interface) they were allowed to stick around.
For a long time we held the line on not having file operations originate from within the kernel (layering reasons). Eventually less sane heads prevailed.
