Here's why:
* Aerodynamics: Our bodies are not aerodynamically designed for supersonic flight. We lack the streamlined shape and strong structural support needed to withstand the immense forces and pressures at those speeds.
* Speed and G-Forces: Supersonic speeds create extreme G-forces (acceleration) that the human body cannot tolerate. Even with specialized training and equipment, fighter pilots can only withstand limited G-forces for short periods.
* Oxygen Deprivation: At supersonic speeds, the air thins dramatically, making it impossible to breathe without a pressurized suit and oxygen supply.
* Heat: The friction generated by supersonic flight creates immense heat, which would be fatal to a human body.
In short, human biology simply cannot handle the extreme conditions of supersonic flight. Even with advanced technology like jet suits, we can only achieve short bursts of flight at relatively low speeds.