How come rectifier diode current waveform is not distorted?
The current and voltage waveforms ARE distorted by the diodes, but you seem to have unrealistic expectations about the expected magnitude of the distortion.

The forward voltage of the diode varies in a nonlinear way based on the current through it, but it's limited to a volt or two at most, depending on the current. Also, the diode blocks current almost entirely until the forward voltage exceeds about 0.6 V. Together, these effects create a kind of crossover distortion near the zero-crossings of the line voltage. Unless you're looking for it specifically, you might not notice it.

Otherwise, the waveform seen on an oscilloscope looks very much like a half-sine curve.