> justify the inordinate amount of money threw at them
They're theorists, you're paying for pencils and paper. String theory may not have produced a theory of quantum gravity yet, but neither has any other line of inquiry.
You are right, the amount of money spent in string theory proposals has been staggering if you take into account the size of the field. For decades competing (or even just non-aligned) research lines have been starved to feed this behemoth.
Particle accelerators have been built since way before any string theory was formulated.
The biggest and most powerful existing accelerator (LHC) has been built to fulfill the high energy/high luminosity requirements to explore the Highs boson energy regime (that has been found) and at most the lightest supersymmetric particles (not found as of today).
The Higgs boson is a cornerstone of the Standard Model. Supersymmetry is an extension to it that does not involve strings.
They're theorists, you're paying for pencils and paper. String theory may not have produced a theory of quantum gravity yet, but neither has any other line of inquiry.