Why do primate fingernails and hair grow only to a set length, but human fingernails and hair continue to grow (until cut or broken).

If we evolved from primates, then surely we should be the same.
We should especially be the same as our last known predecessor - yes?

You need to check your facts and have been misinformed! primate nails and hair grow continuously throughout life just as occurs in humans. they then get worn down and shed due to use.