All is in the title
Why we can't use Xcode on Windows or directly on your iPhone ?
To add to KMTs answer, because xCode is over 10gigs in size and requires system level libraries that do not exist on
either platform. Also, Windows™ is of the devil 🙂... Seriously, though, outside of iTunes®, Apple does not support Windows™
in any way. iOS devices simply do not have the storage, memory or processing power to support xCode and all of its many
system level components. And, in iOS specifically, there is no file system compatible with the xCode environment which means
xCode would not work like xCode on the desktop/laptop if it were ported to iOS.
Because it is two completely different systems, the operating environment is compatible.