They aren't? Because I see this happening all the time. Automated checkout lines, E-Trade and online brokerages, etc. Low-value transactions are handled more and more by electronic "salespeople". All it takes to reach high-value transactions as well is for the clientele to decide they trust machines more than humans, which is not all that unlikely given the many sources of human error involved.
It's possible that humans will be involved in high-value transactions like investment for a long time, since the marginal cost of human labor is low as the numbers get bigger, but it's not impossible to replace; we already have the technology.