It’s a byproduct of the fact that each cohort becomes more conservative with age: https://www.latimes.com/science/sciencenow/la-sci-sn-millenn...

> On average, Americans identify as more politically liberal at age 18 and become increasingly conservative between their 20s and 60s.

See also: https://fee.org/media/14135/chart2_socialism.png?width=100%2...