Why does ammeter or voltmeter measures RMS value
I'm assuming that you are asking about an old-fashioned moving-coil meter.

These meters measure AVERAGE value, not RMS.  An AC meter is calibrated to read RMS voltage with a sinusoidal waveform.

These meters can indicate the exact voltage at some particular point in time IF the waveform is changing so slowly that the meter needle can follow the waveform.  This is possible at frequencies significantly less than 1 Hz.

At higher frequencies, the mechanical inertia of the meter movement acts like a low-pass filter and the needle indicates the average value of the input.