Why An ideal operational amplifier draws essentially no current at its inverting and non-inverting inputs, which means its input impedance is effectively infinite. Real devices come close: bipolar-input op-amps are typically in the megohm range, and FET or CMOS input stages reach thousands of megohms or more. That very high input impedance is what lets an op-amp sample a signal without loading the source, so the gain is set almost entirely by the external feedback resistors. The companion ideal traits are very low (near zero) output impedance and very high open-loop gain.
Watch out The specific figures like 100 ohms or 10,000 ohms look like plausible circuit values, but those are more typical of an op-amp's output loading or of ordinary discrete transistor stages, not the input of an op-amp. Very low describes the output impedance, not the input.
Op-amp ideal: input impedance very HIGH, output impedance very LOW, gain very HIGH.