Because the tests measure knowledge, and schools are intended to impart knowledge.
As an analogy, my physical fitness is totally in my control: if I work out more, I'll build more muscle. But you'd never say "Why are you even going to the gym if your fitness is within your control?" If my local gym is really trash or I can't afford one, I can still do pushups on my own or go for runs, but nobody would argue based on that that gyms are worthless.
Likewise, schools are (or should be) built for students to learn. Students can learn in other ways, too, and any kind of learning will be measured on these tests.
The fact that other means of learning exists, though, doesn't mean that schools aren't important or useful.