Because performance within the audible band is what audiophiles care about. But some folks must only listen to music where all the tones are at 1khz. So this test would be meaningless for them.
1khz is within the audible band. I’m guessing 1 khz is chosen because its harmonics are within a reasonable range. Perhaps a sweet spot. Not sure.
Some people may actually wish to listen to a 1khz tone, but there are probably other meaningful therapeutic influences in play should they wish to do so..
You may wish to listen to a 15khz tone by what you have said. A most interesting anomaly. I don't think it would be very pleasant.