Nature of relationship between atmospheric $rm CO_2$ and temperature

So I had a simple thought:

Venus is ~400C with 95% CO2, and Earth is ~15C with 0% CO2..

Assuming linearity, this equates to 1C for 1% atmospheric CO2.

Why is the relationship assumed to be so much more hyperbolic? There is no feedback loop where a little more CO2 unlocks more CO2 trapped in rocks – save for a little decomposing methane..