Question

why do a heavy and a light object fall at the same time

Answer 1

it is because the gravitational force applied by both the object is same

Answer 2

A heavy object and a light object will fall at the same time to the earth surface of thier will be vaccum.

This happens because at that time only force of $\textbf{gravity}$ acts on both of them.

This force of attraction will act at the same rate on both the object, and hence, it give them the same speed to fall down.