The economic crash was so obvious -- in hindsight.

Big banks such as Citigroup (NYSE: C) and Bank of America (NYSE: BAC) were lending enormous sums of money to borrowers who stood no chance of paying it back. Housing prices were so clearly overvalued. Consumers were keeping spending up only by borrowing unsustainable amounts. How could we have missed it?

Yet so many of us -- even (or especially) the experts -- did.

Earlier in December, I sat down for a wide-ranging interview with famed Wharton finance professor Jeremy Siegel. Here's what he had to say when I asked him why so many experts missed the crash.

Fool contributor Morgan Housel owns B of A preferred. Follow him on Twitter, where he goes by @TMFHousel. The Motley Fool owns shares of Bank of America and Citigroup. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.