Why is the horizontal velocity of a
projectile constant?

Explanation:

I'll try to answer this is clearly as possible without talking in circles. If an object is launched into projectile motion, something was behind it, pushing it forward. Once this launching was done, the launching mechanism was no longer important to the object and the object is free to keep moving forward at that same velocity forever...if that was the only thing at play here. But, because of air resistance and GRAVITY, the object is pulled down and eventually hits the ground. In other words, if there was no such thing as gravity, an object would continue to move forever in the diretion and at the speed at which it was launched.

if they have a finite resultant