There are many reasons behind this. One is the ant mass itself. The mass of an ant is very less compared to other animals. As the mass of an object decreases, its gravitational force also decreases.
Suppose an ant falls from a height. Gravity pulls it down. At the same time the air in the atmosphere exerts an upward force on it. It is called ‘Lift’. These two forces are present in any flying object. It is when the force of gravity is greater than this ‘lift’ force of the air that it hits the ground and associated fall injuries occur.
In the case of an ant, these two forces are almost equal, so the fall will be very slow. When an ant falls down in this way, it will have a terminal velocity depending on its size, shape and mass. Terminal velocity is the maximum velocity that a falling object can attain in its free fall.
The surrounding air exerts a force on the ant that is proportional to twice the speed with which it falls. The terminal velocity of the ant is estimated to be about 6.4 Km/hr. In humans it is 200 Km/hr. The way the ant spreads its legs also influences terminal velocity.
In addition, the body structure of ants is such that they can survive the sudden shock of such a fall. To an ant, traveling through the air in such a state feels like going to the bottom when we fall into water. Not only does it go slower, but it also doesn’t cause injury from falling down.