The same reason power lines are 4000 to 14400 volts and converted down to 120 volts. High voltage lines can be 100000 volts or more.
Efficency. less loss to heat. and smaller wire size to carry the same amount of electricity.
The amount of electricity wasted to heat increases as the voltage goes down.
Also as a wire gets hotter the amount of electricity it can transfer goes down.
So as the wire gets hotter voltage drops. When voltage drops heat increases. When heat increases voltage drops.
It is a vicious cycle that has to be designed arround.
If the power supply converted down to 1.5 volts the wires coming out of the power supply would need to be arround 8 times larger to carry the same total amount of electricity without overheating.