Hail damage at car dealerships in my area is so expensive that the dealers have added coverings for the lots (pretty sure highly encouraged by the insurance companies), and they are most definitely not nets. Nets would have been cheaper, so that should say something about their effectiveness (or not)
Ascetically, nets get dirty and are hard to clean, have birds on them, things grow on them, and they look gaudy to some.
Hail protection nets are a COTS product. Full stop. Properly-designed nets are exceptionally strong; that's why we make "bulletproof" vests out of them (that's all they are, just a fiber net with a very tight weave).
Just because your local car dealership(s) didn't opt for hail protection nets isn't on its own evidence for or against their effectiveness. There are a great many factors that go into such a decision and, unless you were privy to the decisionmaking process itself, whether or not nets were considered -- much less their effectiveness -- is pure speculation on your part.