Take a look at this chart, the Circle of Fifths: https://en.m.wikipedia.org/wiki/File:Circle_of_fifths_deluxe_4.svg

5 sharps is B major, which is the same notes as if there were 7 flats/C flat major. So even though 5 sharps would be the same notes as C flat, it wouldn’t be as accurate to call it that (instead of B major), because C flat would have 7 flats.