Many times you haev a 30-degree slope at the end of the runway tile, and you cannot land early or land too late for fear of crashing into it.
In a perfect terrain that's easier, but a lot of fields are far from perfect. Sometimes you don't have the luxury of stopping at the end of the runway just as you pass it, and sometimes you don't want to go in "too hot" depending on the plane you're flying or the unfriendly terrain aroud the runway tile.
Oh, and let's not forget those damned trees that like to watch all the planes come and go, right at the ends of major runways.