If humans can only hear up to 20 kHz frequency sound, why is music audio sampled at 44.1 kHz?
The sampling rate of a real signal needs to be greater than twice the signal bandwidth. Audio practically starts at 0 Hz, so the highest frequency present in audio recorded at 44.1 kHz is 22.05 kHz (22.05 kHz bandwidth).
Perfect brickwall filters are mathematically impossible, so we can't just perfectly cut off frequencies above 20 kHz.  The extra 2 kHz is for the roll-off of the filters; it's "wiggle room" in which the audio can alias due to imperfect filters, but we can't hear it.
The specific value of 44.1 kHz was compatible with both PAL and NTSC video frame rates used at the time.


Note that the rationale is published in many places: Wikipedia: Why 44.1 kHz?