Why high quality broadcast studio monitors use a middle range AB amplifier?
Monitor speakers are made like any other product, and therefore made to cater for a certain type of customers. Monitor speakers are a bit of a niche market, which means that the manufacturer has to maximise the amount of profit on the amount the customers are willing to pay for.

The engineer's job is to make the best product possible for the lowest cost, and here is where compromised come into play. In this case the lowest cost is defined by how much the sales department allows you to spend.
This budget will define the components that can be used in this product, so if the best transducers that can be fitted into the monitor have a distortion of, let's say 1%, there is no point in selecting an amplifier that has a distortion of 0.1%. In the same way selecting an A class amplifier, will require a much larger power supply which will eat into the already small budget.

It is all down to compromises, and the engineer's job is to design the best product that a limited amount of money can buy.

An experienced engineer will also choose the components in a way that speakers will have a frequency response that is as linear as possible to the human ear, and therefore compensating for any of the flaws of the other parts in the system.