Ultrasonic sensor

Why we just use ultrasonic waves in ultrasonic sensor?
I know that we can't use sound waves because this is harmful for us and ruins the privacy but why we don't use sub-sonic waves?

Why we just use ultrasonic waves in ultrasonic sensor?

Because that thing is called ultrasonic sensor. Otherwise it would be called subsonic sensor.

There are subsonic sensors but ultrasonic sensor are usually distance sensors and the physics of subsonic waves are not ideal for distance sensing.

Sub sonic implies very long wavelengths, it would then be difficult to resolve small distance , plus the transmitter and receiver would have to be physically large and consume more power to operate .

Subsonic wavelengths are 17 metres or more, no use for sensing on smaller scales.
Most ultrasound transducers use 40kHz which is about 8mm wavelength, good size for
sensing the environment to fair accuracy. wavelength = wave velocity / frequency.

Furthermore a subsonic transducer would need to be large, like its wavelength, to
function well.