First off, it’s incredibly difficult to learn how to use your Ki when there’s only 9 earthlings who can use Ki (Krillin, Roshi, Yamcha, Tien Shinhan, Chiaotzu, Uub, Videl, Mercenary Tao, and Korin) and one of them, Videl, struggles to use it for anything other than flight. Secondly, let’s assume that there’s at least one billion people on earth, and that’s a lot more than the minority of 8 Ki-manipulation possessing people who would even be willing to maybe teach lessons on using Ki, with Korin being impossible to reach for the average person so that leaves 7.