if we add the two velocities then it wont be called the average velocity.
It is basically used to describe the rate of making a complete displacement in a particular time.
Eg. for a body going 5 m in one direction with a belocity of 1m/s and coming back directly back with a velocity of 3 m/s then its average velocity is 0 as its total dispalcement is 0 but if we find the average using the usual formula of adding the two velocities and then deviding by 2.
This is so because it is defined otherwise as total dispalcement by time taken