> What am I missing?

Perhaps the fact that Brexit was at least partially driven by xenophobia, and no politician will gain popularity by announcing "We are making it super easy for foreigners to come work here!" (and yes, even highly skilled ones).

Yes, I think the UK will end up carving exceptions everywhere they can to keep Brexit alive from a political standpoint, but dead from a practical sense. Right now, the fear of the unknown is what's most likely pushing people not to move to the UK anymore. In five years from now the situation should be clearer I'd say. I personally hope they'll open things up again but we'll see!