Some clients can be scared off by a high hourly rate when in reality it may well work out cheaper for them than paying a flat amount per month. But then again, even certainty in the billing cycle has its cost.
There are bookkeepers of all different skill levels out there; we have all seen this when receiving handovers or conducting rescue jobs. As mentioned by others, the rate needs to allow for overheads. Therefore you would expect someone who works from home to charge less.
Also if a bookkeeper is registered as a BAS/Tax agent, has an accounting degree, a CA/CPA qualification and of course sufficient industry experience, you would expect to pay more. Like a lot of things in life you get what you pay for, to a point.