If our prints don’t change, why do we have to redo them at any point?

Without doing any statatory reasearch to see if the reason is actually addressed, I would suppose that one reason for the requirement could be to ensure that the person alpying for the renewal was, in fact, the same person who got the orignail licence.  I can see such a policy being rationally related to the State's interest of prtecting children and, therefore, justified under that rational.