Not disputing anything that's been said or condoning the military's reasoning, but to answer your question: The military is downsizing, so first of all they're not looking to find a way to retain more personnel (again, not condoning), and, let's face it, they were never good at implementing change to being with. The tape test is the cheapest both in terms of equipment and training required. Even if only some bases had more reliable methods, there's still a cost involved, plus the cost of transporting soldiers from other bases. It is just not cost efficient for the military to install that equipment and then pay to drag every noncompliant soldier there and back, for the sake of the one or two out of hundreds or even thousands for whom it will actually make the difference between passing and failing.
If they really wanted to, I'm sure they could develop a "you get a dunk test if you have x PT score/commander recommendation/whatever" but there would still be some who feel shafted and, again, the military isn't looking to spend more money just to retain more personnel. And so you have some people who pay the price.