Not everyone is able to leave their foreksin permanently retracted.

Some are too long and insist on going back.

Some are too loose/wide and the glans coronna isn't wide enough to keep the foreksin behind it.

For a number of males, the foreskin becomes naturally retracted all the time after puberty without them having had to "train" it consciously. Foreskin stays retracted after erection longer and longer and eventually just stays retracted all the time.