Because the freezing point is at a different temperature on the two scales. If both used 0 for freezing, then half of one would be half the other. But because of the offset, they do not stay linear with each other.

In other words, if (degF = A x degC), you would see the kind of relationship you posted. But instead, (degF = A x degC + B), where A= 1.8 and B = 32. That “+ B” throws everything off.