Since the endodontic instruments are placed inside the root canal and root canal procedure done under anaesthesia, hence it does not effect. If same instruments worked without LA the patient complaints unless the tooth is nonvital

Even in non vital teeth Periodontal Ligament is not sensitive in Root canal space in comparison to outer surface towards Gingiva

Because these instruments have high speed annoying voice probably to cause nervous for the patient.

Nonvital teeth do not need injection as long as you do not put pressure on the tooth. With the high speed instruments you can drill the tooth with feather touch without putting any pressure. The pressure on the tooth causes pain. Once the pulp chamber is exposed the pus comes out and the patient gets immediate relief.

I guess it is more psychologically than scientifically ( the facial expression )

also the pressure on the tooth causes the feeling of pain .

An ideal irrigant should reduce instrument friction during preparation, Facilitate dentin removal, Dissolve inorganic and organic tissue, Penetrate to canal periphery, Kill bacteria and yeasts and least irritating to the periapical tissues. However, there is no one unique irrigant that can meet al.,l these requirements, even with the use of methods such as lowering the pH, increasing the temperature, as well as addition of surfactants to increase the wetting efficacy of the irrigant. More importantly, these irrigants must be brought into direct contact with the entire canal wall surfaces for effective action, particularly for the apical portions of small root canals.