Why is Uber allowed to test its software for free on public roads ? Why doesn't it use a purpose-built test-track of its own, where it must prove that its softw

Why is Uber allowed to test its software for free on public roads ? Why doesn't it use a purpose-built test-track of its own, where it must prove that its software won't kill people (using robotic crash-test dummies to avoid) before it is allowed to use public roads ?