I'll try to explain it with simplified physics. Let's assume for a moment that the projectile is fired 50 ft off the surface at a speed of mach 8, or about 9k fps. If we assume no resistance vertical or horizontal at a 50ft drop the projectile is travelling downward at 55 fps by the time it hits the ground. As you can see, the horizontal velocity is still orders of magnitude greater than the vertical velocity. As you start to deal with curvature of the earth your drop distance will increase, but not enough to overcome the 9k fps in horizontal velocity any time soon. The horizon is 3 miles off, or 16k ft. Less than 2 seconds of travel time for our projectile. Of course, there's always going to be resistance, so at some distance the horizontal velocity line will cross paths with the vertical terminal velocity, but I doubt that happens at any practical firing distances.