Have you ever seen a runner (or are you a runner or were you a runner) who OVER-STRIDES? Overstriding is when you see a runner landing heel first with that heel first landing happening in front of the center of gravity? Overstriding is actually a biomechanically inefficient way to move and can result in hamstring injuries and slower movement. Think of it like putting the brakes on while you try to accelerate.
For the horse: A flat or nearly flat landing is actually the most biomechanically efficient way to a horse to load and go as well. Perhaps the tiniest of heel first landing. But a prominent heel or toe first landing is a no-go for horse and human alike.