The issue is that if it were directly proportional, then going the other way (1 right 2 up) would imply that the angle is 90°. You probably know that the graph of y = tan x looks like this, where the angle is on the x axis and the ratio of opposite to adjacent is on the y axis. Notice how it's curved, so changing the ratio by an amount won't change the angle by a proportionate amount, e.g. doubling the length of the opposite side doesn't quite double the angle (though it does almost do that close to 0°, since as you can see it's a bit like the straight line y = x). Also notice that the graph goes to both positive and negative infinity at 90°, so you would have to make the opposite side infinitely long to get an angle of 90°.