Because it would cost Apple a substantial amount of money to make it possible to build your code for iOS on other platforms in any sort of supported fashion. So all the stuff they invest very substantial amounts of money into creating and maintaining are only going to be licensed for use on the hardware they sell under the latest operating system it runs. Asking for more is asking them to spend more money without any evidence of a commensurate return.

If you can produce a properly structured submission to the App Store ingestion pipeline without using a Mac, then I doubt Apple will care. But that doesn’t mean they’re obligated to help you, and especially not to spend millions of dollars to do so.