Because it's not contextually the same thing. You could be along the beach and they give you directions to go "upland" (e.g., towards the mountain). But you might not be near a hill at the time. "Turn uphill" would usually be if you're already on an incline (or a slope) because you could go uphill or downhill. I don't think I've heard anyone say "uphill" while on flatland (probably something like "go up those hills").