Because they don't know better. They've been circumcising their boys for so long, they don't even know what a foreskin looks like.
As a population, they've forgotten all about foreskin, its role in the sex act, its role keeping a newborn baby healthy, its role supporting the structure of the penis during erection. They don't know about the pleasure it provides.
Parents are swayed by fear mongering, folklore, outdated medical practices, trapped physicians, out-of-date baby books, social custom, and a shroud of secrecy that makes open discussion difficult.