Nobody is preventing string theory from being tested. If you know of a way to test it, please go and do so, you'll get a Nobel Prize for it because nobody else has thought of a way.
That's exactly the problem - spending decades studying a theory without any idea of how it could be tested (with devices that could, say, fit into the solar system) is not how science should work.
Perhaps we should simply study other fields, where experiments are at least in principle possible. The ancient Egyptians didn't have to study quantum chomodynamics in order to advance science.
