Why? Draw the line backwards, and in a couple of decades you are down at 0 IQ. That's clearly absurd, you can't draw any conclusions of IQ significantly before 1950 from how the line behaves after 1950.
And that’s because IQ is a statistical distribution, not an absolute measurement of intelligence.
If everyone suddenly gets twice as smart as before, nobody’s IQ changes.
the 'IQ' people conveniently ignore how the IQ test is such a poor measure for intelligence & resourcefulness