> But then why didn't they cut it into 10 pieces - https://etc.usf.edu/clipart/40600/40610/pie_01-10a_40610.htm ?

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)