[zachlipton]

Two reasons. The first is that these people often don't look like salesmen. They come in appearing to be providing impartial advice, and all the incentives, commissions, etc... that bias their advice are kept secret. The other problem is that these guys are often pushing complex products to an uneducated public where small details can make a big difference. A small change in an annual fee for an investment can, thanks to the magic of compound interest, easily add up to huge differences in investment returns.