Why is it 'impossible' for a U.S. citizen to move to the UK?

Every time I ask a question about moving to the UK, I am told that it is almost impossible to do as a U.S. citizen! My husband and I have a combined yearly income of approximately $275,000 a year, and I know that pounds are worth more than US dollars. I know that they look for skilled workers, but my husband is a surgeon and I am a Nurse Practitioner. My mother lived in the U.K. for years with her old boyfriend before moving back to the U.S. So why would it be 'impossible'?