Building for Windows on Linux

I’ve been programming for roughly thirty years, and every Windows application I’ve produced in the last five I’ve built from Linux. So, I know it’s both possible and relatively simple in most cases. I’m led to wonder what it is specifically which is preventing Unreal from allowing for this?

I can only assume that it has something to do with some awkward Microsoft license from days of old. Something MingW can’t effectively step in for.