“Royals did it, wealthy families here did it to keep money and/or land from being broken up outside the family. It wasn’t always a sign of ignorance, it was done for practical reasons.”
Sometimes there were problems. That Habsburg lower jaw, for example. Then there was Charles the Bewitched.
Plenty of articles online about Muslim inbreeding. Keeping the bride-price in the clan is one motivation.
Never said there wasn’t a potential price to pay with some progeny, just that it’s been done and intentionally, not out of ignorance, and not always with disastrous result. Generation upon generation of it done out of ignorance with no consideration does lead to widespread problems though, as does generation after generation of it done due to some sort of closed environment, whether that’s lengthy isolation or clannishness.