You don't necessarily make less.
For example the royalty on this book for a purchase would be 35 cents
https://www.amazon.com/gp/product/B07DCFWNYD/
Whereas a KU read based on the approximate per page rate and a guesstimate at the KENPC value would be significantly more.
I've read that book, it's not terribly great, so I wouldn't recommend it. But it wasn't bad enough for me to abandon it.