Must be a state rule. My grandaughter passed hers at 16 and got her diploma in the mail in a week and used it to enroll in Nurse Assistant courses at the 2 yr college locally.
It is a state rule. States have any number of weird rules to keep their control over minors. The goal is to keep them in public school as long as they can.
In my state I wanted to hire someone to clerk in my store a couple of days a week during the summer. There were several young people who were interested but the local school board, who does not like me, refused to give them work permits. I ended up having to hire from the next county.