Data: FactSet; Chart: Andrew Witherspoon/AxiosAn index that tracks the major banks is underperforming the broader market. And not just in light of the stock market rout it's underperformed for most of the past year.Why it matters: The higher interest rate environment that's spooking investors is supposed to be good for banks. They can charge consumers more for lending, boosting profit. But the advantage is not showing up in these companies' stocks.Why the slump in bank stocks? Analysts are pointing to banks' loan businesses, which has not been as strong as expected in a booming economy.Last quarter, embattled Wells Fargo reported declining loan growth and recently warned at an investment conference that this quarter probably would not be any different.What's going on: It isn't just that consumers are taking out fewer loans for fear of higher interest payments. There is more competition for lending from companies like Quicken Loans to private equity firms. Banks are facing probably the "largest threat" from non-bank loan originators, Mark Doctoroff of MUFG's Financial Institutions Group told Axios.Yes, these non-traditional lenders are backed by traditional banks, but it's not showing up as typical loan growth as it has in the past.Another factor: post-financial crisis, banks have been more disciplined and diligent about the quality of loan and origination, Doctoroff points out.What's next: A clearer picture of the health of the banks' loan business. J.P. Morgan, Citigroup and Wells Fargo report third quarter results today.