Yes, a Power of Attorney may do everything a family needs. However, in other situations, a Power of Attorney may not be as enforceable as the family hopes.
Adult Guardianship is put into place by a court Order. If the Ward decides one morning that they no longer want their court-appointed GOP (presumably a family member) as their guardian, they do not have the power to change anything. Only the Court can modify powers. This protection is what drives many families to file a guardianship petition.
A saying that flies around the courthouse, time after time, is 'I know I waited too long, I just wasn't sure it was the right time.'
Guardianship is a protection, not a punishment. Hopefully your loved ones have an estate plan that has been executed and ready to be activated. Sometimes, it's too late. Other times, even with an estate plan, Guardianship is necessary.