Sallen Key topology input and output impedance
The input impedance of an opamp is that between the inputs. In the Sallen Key circuit, the amplifier is used as a follower (the other input is connected to the output), so the input impedance is 'bootstrapped out' and becomes very high within the bandwidth of the amplifier.

Even a single transistor used as an emitter follower gives very little error (except for DC offset of course) in this circuit configuration.