Because the foreskin isn’t just a useless spare part. From an evolutionary perspective, it serves a purpose.

Think of it like buying a car and modifying the transmission. The car still performs it’s primary function, but in a way that comes with trade-offs.

Circumcision is the same. Removing the foreskin (modifying the transmission) might improve some things like hygiene, but it will also completely remove the penis’ natural barrier to protect itself from excess friction