Sorry that you're having to go through all of that. There are a number of reasons, which are almost never the fault of the individual.
ElPaso Pilot is correct. Some companies may still extend credit to someone impersonating you anyway, but this should be the exception, and as Clip4 points out could easily be an insider threat or external compromise of a company that won't be prevented by any outside solution.
For those that haven't had a problem yet, two things you can do to reduce the odds of having the problem are: First, and probably obviously, get credit from as few places as possible. You need a bank, a mortgage, a credit card, sure. But having a bunch of random store cards just increases the odds you'll be hit. Sure, Citibank might be hit, but you can be pretty sure they have better security than the typical discount store. If they do, it's way better if all the bad guys get is your credit card number, not your social and address. Second, if you have a business don't use your personal SSN as your business tax ID. Get a different number to use for whatever permits and licenses you have, and for the tax ID your employee has. People setup businesses with their own SSN all the time, but it's not a good plan.
No solution is perfect, as states and the feds are terrible at security, in general, and unfortunately you have to give them sensitive information in many cases.
Eventually this will go away, but not until SSN stops being used as a secret code.
