Here's why:

* Vertical motion: As a projectile travels upwards, gravity constantly acts against it, slowing it down.

* Highest point: At the highest point, the projectile momentarily stops moving upwards before it starts falling back down. This means its vertical velocity is zero.

* Horizontal motion: The projectile's horizontal velocity remains constant throughout its flight, assuming no air resistance.

Therefore, the total speed of the projectile at its highest point is equal to its horizontal velocity.