In my case it's because they appear to be funded from the public purse yet do no public duties.
In the 20th century, the British monarchy reinvented itself with a strong public service ethic, such that the quid pro quo for being a royal person is spending your time on public duties, doing charity work etc.
Yesterday I attended the unweiling of a war memorial by the Duke of Kent. He is 78, possibly not in the greatest health (he arrived at the memorial by golf buggy), but is still doing a steady round of public duties, mostly out of the limelight. His three children and young adult grandchildren do no public duties, but carry on their lives as private citizens. The York girls seem to be trying to have their cake and eat it, which is never popular with the British.
Ann