Landing it horizontally would require the rocket to be able to coast horizontally, meaning it would have to have wings. I can imagine wings would make the whole

Landing it horizontally would require the rocket to be able to coast horizontally, meaning it would have to have wings. I can imagine wings would make the whole thing more complex but even if they nailed that, imagine what it would have to do on the descent. You basically have two choices, either fall straight down and try to control spin, or coast down, trying to control horizontal and vertical speed like an aeroplane. I have the suspicion that when dropping from space it's very hard to get into coasting mode since the air is so thin. However it is, a rocket that simply falls down instead of coasting like an aeroplane is so much less structurally complex, the only real challenge is controlling its landing which modern computers and their rocket engine make feasible.