With exception of disaster loans, the SBA is not set up to lend. They don't have the infrastructre to do what hundreds of small banks do collectively, which is to screen, underwrite, fund, and service all those loans. It's probably more accurate to think of the SBA as the borrowers rich uncle who agrees to guarantee the debt, as opposed to thinking of the SBA as a lender.
As Phanio mentioned, the SBA only pays the lender once a loan is 60 days past due. So when someone says there are $500 Million in SBA loans out there, it doesn't mean the SBA has laid out $500 Million.
The mission of the SBA is to incent banks to lend to borrowers who do not meet traditional lending criteria. The premise is not a whole lot different than "sub-prime" mortgages, which also were made to borrowers whose repayment ability was in questions. The big difference between the SBA program and sub-prime mortgages is that if the business flourishes...it created jobs, puts money into the economy, and all the stuff.