All SAR ADCs have a sample and hold capacitor which must be charged by input to the ADC. High impedance inputs can't quickly charge the sample and hold.
I couldn't help but notice this generalization. An SAR ADC does not have to have a track and hold amplifier in order to be an SAR ADC. Early chip ADC's had separate SHA's and in fact such IC's are still in use and production. I'm not trying to start an argument, just pointing out a fact that has maybe fallen through the crack due to the rapid rise of technology.