Probably because lenders have to pay 5% interest on demand deposits in the current interest rate environment and would lose money if they lend at a 3.75% index rate.

I have been in banking for over 25 years and have been running Commercial Lending X, a Commercial Loan Brokerage and Bank consulting company for fifteen years. I have never once in my career seen a lender use the SBA Peg Rate. I think ^redacted‌ is largely right. Demand deposits costs too much. The loan would not be profitable. SBA 7A loans tend to be higher risk as well, which is why usually traditional banks will not do the loan. Banks are not going to charge such a low interest rate for the capital they do have at risk on the loan, even though it is only 25% of the loan amount. Also, the market is not there. No one is doing it. Lastly, most lenders sell the SBA guaranteed portion of the loan on the secondary market, and they would get no return for a rate that low. If you are looking for it you are not going to find it.