The slopes don't let you do it even if we had sprites. Floors/ceilings and walls are handled entirely differently by the game. Slopes are angled floors or ceilings, not walls. This works fine for normal gameplay, but doesn't allow for loops at all. You'd run up the bottom half of the loop and just stop dead when it hits 90 degrees, because that's a wall and you can't run on walls. The slopes honestly don't work very well beyond 45 degrees anyways.
Not to mention the fact that in 3D, you can just walk through the center of the loop without going around it anyways. What would even be the point? Even if you went through all the trouble to rewrite massive parts of the engine to make it work, all you've got is something pretty that's completely impractical. Our camera would absolutely cry going through something like that, and nobody would ever actually go around it except to see if they even could.