* Most schools don't use a 5.0 scale. The vast majority use a 4.0 scale (A=4, B=3, C=2, D=1, F=0). A 5.0 would imply a system that goes beyond an A, which is rare.
* Weighted GPAs can exceed 4.0. Many high schools use weighted GPAs, which give extra points for honors, AP, or IB courses. In these systems, a student taking many advanced courses could easily achieve a GPA above 4.0. So, a 5.0 weighted GPA is possible, though still relatively uncommon.
In short, a 5.0 unweighted GPA is practically unheard of. A 5.0 weighted GPA is possible, but still unusual. The key is understanding the difference between weighted and unweighted GPAs.