Well, for one, you may have noticed that MS put in the work, over years, to make .NET the same implementation across all three platforms. So that’s at least one pillar of impossibility removed.
Except ODSP/EXO/Dynamics don't use CoreCLR -- they use good ole fashioned .NET Framework. Those products with their 20+ year old code base would require a full rewrite.
That doesn't make economic sense.