If we cannot hear above 20kHz why do engineers insist on building amplifiers with ever higher bandwidths?
While I can't tell you why designers other than our own like to extend amplifier bandwidth into the ultrasonic regions I can explain why we do. It's called the Rule of 10.
The rule of 10 is a lofty engineering goal that simply states we should strive to build products with 10 times the required average use case. So, for example, if we want to make sure we can deliver flat frequency response out to 20kHz then we should try and extend the amp's bandwidth by a factor of 10, or 200kHz. In the same vein if we want to be flat to 20Hz we'd make certain to extend that by a factor of 10, to 2Hz.
We don't always get what we want nor do we use the Rule of 10 as a strict formula, only a guide. The reasons for this are simple. By stretching the parameters beyond requirements we gain headroom, staying within the comfortable bounds of an amp's abilities. Taken too far in any one direction this goal can actually make things worse. For instance, demanding too much ultrasonic performance can require design changes that have negative impacts elsewhere.
The Rule of 10 is valuable advice if it is kept in balance and harmony with the rest of the design.