Why is kicking a football an example of projectile motion?
Actually whilst the ball is in contact with the foot it is not a projectile. A kicked football is an example of a projectile (i.e. after it has been kicked).
A projectile is an object that moves under the influence of gravity, what that means is that it's weight is the only force that acts upon it. In reality there is a drag force too, but that is frequently ignored for the purpose of projectile calculations.
Whilst being kicked the ball has a normal reaction force from the foot acting upon it in addition to its weight. So that does not count as a projectile. After being kicked the ball only has its weight (and drag) acting upon it, so it is a projectile. Whilst in flight the ball will continue with constant horizontal velocity (no horizontal forces) and experience a constant downwards vertical acceleration (due to its weight).