You can use a clamper to produce a waveform that has an average value that is approximately the peak value of the AC waveform but you'll still need a diode and a capacitor to smooth that to a DC supply capable of being useful. Then you'll get a smooth DC supply voltage that is approximately the peak-to-peak value of the AC waveform.
Incidentally, this method is used in the cockcroft walton multiplier: -
So can you please provide why cant we use the clamper for output DC
You can as per the example of the cockcroft walton voltage multiplier.
Another example of using a clamper to get a DC level is when dealing with a small AC signal such as from an audio line out: -
With just a few hundred mV RMS to play with, a DC level can be produced that is sufficient to turn on a MOSFET (such as a BSH103) and drive a relay. The two diodes and two capacitrs form exactly the same circuit as the 1st stage of the cockcroft walton multiplier shown above.