Why is the absorbance versus concentration curve for a substance rarely straight for all concentrations?

In fact, it should be a straight line but the curve is rarely straight when the concentrations are either to high or to low.

This is due to the accuracy of the spectrometer. If concentration is too low, the amount of light transmitted is near 100% of the incident light and the electronic device can not measure it precisely. In an opposite way then the concentration is too high, the amount of light transmitted is too low and can not be measured accurately. However Beer-lambert law holds.