Basic question

Why Single look complex data is called single look?

Because it is not multi-looked yet. SLC represents the radar image after compression in slant geometry (different azimuth and range resolution) and contains both phase and amplitude.
Multi-looking aggregates information to produce images with equal pixel spacing by merging information. This can be done in the space domain (several pixels are combined in range and azimuth direction) or in the frequency domain (several sub-spectral band widths are merged) to create an image in ground range geometry with reduced speckle effect.

I made a comparison of product levels in table 33 of this document.