I'm a banking employee, and did a stint in our financial and accounting services division, the area of the bank that handles payments to the bank. This isn't specifically inter-customer transfers, but rather payments that the bank does to vendors or that vendors pay to the bank.
As has been noted in other comments in the thread, before payments go through there are several controls that need to be passed. Most of these are automated (checking if the account being paid into doesn't belong to an ISIS affiliate, seeing if someone with a $2000 salary isn't suddenly getting a $250,000 payment, etc). However, these still need to be manually approved by at least 3 different people (in our world). This doesn't mean that 3 different people are looking at your payment before clicking "Accept", but more that 3 different people are looking at a list of payments with all the details listed before batch loading them to the payment structure. They're just looking for anything that looks "out of place"; so either big payments, irregular payments or maybe something weird like someone paying themselves 1 cent from a million different bank accounts. 99.99% of the time, they don't pick anything up because the computers have already picked up the problem, but that's not the point.
The point is that now one bank employee can't just grab money from wherever and dump it into whichever account they want. Requiring 3 randomly assigned people to perform the check ON TOP of the computerize system reduces the (mostly internal) fraud rate to a level that most banks can live with. In other words, their risk and insurance are low enough because of this that they are willing to risk it happening, weed out 3 bad apples with one stone and then moving on.
What this also ensures is that someone who hacks into the system over the weekend cannot automatically pay themselves a million bucks, go to cash machines to withdraw it all and the bank (and maybe even customers) sit with their thumbs up their assholes come Monday morning.
tl;dr: the actual payment doesn't require a multi-step human intervention. Preventing fraud, collusion or an inside job does.