The lack of data migration may be related to recent EU lawsuits that essentially forced them to delete all data that has been collected without legal reason. I guess it's hard to impossible to separate this data from "clean" data collected outside the EU after several rounds of aggregation.
So, clean cut seems like a good idea.
The data migration might stem from the fact that the underlying data model is so totally different, that it just might not be possible to migrate anything.
GA3 and GA4 are so different, it isn't like the US and Russia, it is Earth and Jupiter (not in terms of impact or size, but just vastly different).
As the most dominant player in the market, they could do this (and to a certain extent still can). But for how long?