This phenomenon is not unique to test prep. I'm willing to bet that there are more Spanish teachers (and therefore a better chance to find great ones) in Los Angeles or Tuscon than in Boise or Cheyenne. But since students usually go to school near where they live, they have to settle for what is available. But imagine an online learning platform that let a student in Bozeman, Montana take Spanish from a teacher in Buenos Aires, English from a teacher in London, calculus from a teacher in Moscow, and art from a teacher in Paris all in the same day? I think it would be interesting to learn about the Vietnam war from a teacher in Saigon.
When people talk about online education, they tend to highlight the isolation students experience. I think it can be quite the opposite...it can be a more social experience than sitting in a classroom a few miles from home with an instructor and peers who likely share the same background as you.
If you've ever worked with parents in this position, you realize the potential danger of DIY education. These parents are embarrassed because they know their kids need education to get ahead, but because they aren't educated, they don't know how to do it. They develop bizarre ideas about what is and is not important. My wife tutored for a poor family that was working double duty to send their son to an awful private school. The school was worse than the public schools in the area, but because it was "private" they felt their son must be learning more. Many online academies are like this: they make you feel narrowly knowledgeable and valuable, rather than introducing you to painful humility the way a top-notch research program tries to.
The time sounds about right for something innovative to change the game; it's just a question of having the will to do it. We'll try a few things, see what works, and iterate.