I believe that is more or less a western bias.
That is the Egyptian royal practice even before the Ptolemies. To them that is normal. Since their civilization is older than us, who we are we to say that is abnormal when they were practicing that when we were all barbarians.
If you look at it, our practices are probably the ones abnormal to them. It is like a difference between a society that accepts only monogamy and society that accepts polygamy.
Except there is completely objective genetic reasons why heavy duty incest in an absolute monarchy is terrible practice. Longstanding cultural differences doesn't actually make something a worthwhile idea.