Ever tried cutting a cake? It’s a lot easier to visually judge half of a circle segment. You’d need a compass to get accurate tenths (or fifths) and I imagine it is generally frowned upon if some tenths are a lot smaller than others (happens a lot with cake)
8 pieces is easier for a human than 10. You halve it, then again, then halve the remaining quarters. For 10 you'd need to work harder to get the angles right and that's a lot harder.
Ever tried cutting a cake? It’s a lot easier to visually judge half of a circle segment. You’d need a compass to get accurate tenths (or fifths) and I imagine it is generally frowned upon if some tenths are a lot smaller than others (happens a lot with cake)