It's like the US refusing to convert to the metric system. In fact, there is no reason we couldn't just use the metric system for shoe sizes, but we've inherited these archaic systems and there is resistance to modernizing them.
Coz the size of their feet is diff ;)
Simple as that!
To confuse the masses. Same thing with pant sizes, shirt sizes and dress sizes.
old tendencies like the shirt buttons on different sides the numbers seem more attractive to people as they are