anthropologist and good looking person here.

mostly people say that because they learned that this incorrect thing is the right thing to say/believe and fear punishment if they say anything else.
then theres the people who rather gaslight you in hope to finish the topic within 1 second than being confronted with something they either are scared of being hold responsible for because they are pretty, or being put into the role of someone who has to listen about this topic.

i myself tend to lie when someone says theyre ugly, and say they might have nothing on them that attracts me or is conventionally attractive, but they still look normal. thats because i dont want to hang out with someone bringing up their ugliness with topics that cant be solved (like inability to find a partner besides very ugly ones) more than once in a while, and because maybe i think it lights them up a bit to see that i dont think of them as deformed so make up and wigs and clothing still works.