Why don't you plot transmittance vs wavelength?

Answer 1

Sometimes you do in fact. In IR spectroscopy, the transmittance is commonly plotted instead of absorbance.

However, in most types of spectroscopy, the absorbance is plotted instead. This is due to Beer's Law, which states that the absorbance is directly proportional to the concentration of the sample, the absorptivity of the sample, and the path length (in contrast, the transmittance is exponentially related to those things). Because of the much simpler direct proportionality, absorbance is preferred in many cases.