We're all used to it - to change it would take far, far, far too much effort. Can you imagine? Having to design it, get everyone to agree on it, manufacture it, replace all the existing keyboards, allow for secretaries and typists to get used to the new layout.... oh jeez, that's far too much effort.
Other keyboard layouts COULD be more efficient. But why rock the boat? The current layout works just fine, the placement of the keys is standard so anyone can use any keyboard anywhere, and we're all used to it. Plus, it's easy to use. I mean, court typists have extremely bizarre keyboards they use to record the court proceedings, but it takes years to train to use effectively. We don't want that - let's just stick with what we have. It's not like we NEED to type at 100 words a minute all the time/