Why the emitter is earthed in the Common-Emitter configuration of a Transistor?
"Earth" or "ground" in this case just means the point in the circuit the designer decided to call "zero volts", and use as a reference when measuring voltages elsewhere in the circuit - it does not imply an actual connection to the earth.  I'd prefer to call that point "circuit ground", or "common", rather than "earth".

The important thing to notice in a common emitter circuit is that the emitter is common to both the input and output sides of the circuit