Why input resistance of voltage amplifier is infinity?

For the successful amplification of the input signal the opamp should have ideally infinite input impedance . It should act like a buffer amplifier

BUFFER amplifier--------------------->
1.input impedance infinity
2.output impedance zero

the reason is that
Any signal source will have source impedance
for the signal not to get lost and dropped across source impedance we ideally insert infinite impedance in series with it which makes the whole drop across the infinite impedance but not across the source
similarly at the output zero impedance is used where in no part of the signal should be left behind in the op amp as a drop