Numpad is redundant part of the keyboard, added to make basic calculations (addition / subtraction / multiplication / division) faster. Studies show that approximately 90% of the population is right-handed. Thus, to maximise its utility, numpad is placed on the right-hand side which is more convenient for your right hand.

Placing numpad on the left-hand side would make it inconvenient for the right hand. So you'd have to use your left hand as you suggested, but that hand is less skillful when performing tasks, and so that would defeat the purpose of numpad itself which is to make the basic calculations faster.

Numpads are on the right-hand side to make basic calculations faster for majority of the population.

Numpad is redundant part of the keyboard, added to make basic calculations (addition / subtraction / multiplication / division) faster. Studies show that approximately 90% of the population is right-handed. Thus, to maximise its utility, numpad is placed on the right-hand side which is more convenient (closer) for the right hand.

Placing numpad on the left-hand side would make it inconvenient for the right hand. So you'd have to use your left hand as you suggested, but for majority of the population that hand is less skillful when performing tasks, and so that would defeat the purpose of numpad in the first place which is to make the basic calculations faster.