Because our color perception is subjective/psychological and doesn't correspond to any mathematical models. Also, the LAB colorspace can't be produced on RGB monitors. In fact, many LAB colors are purely imaginary.
Furthermore, CIELAB is considered the most basic of the CIE "color appearance" models, which have been gradually developed over the years to better model perceptual color. Newer color models include S-CIELAB and CIECAM02, both of which use human contrast sensitivity functions to better model human color perception. A more recent and even better perceptual color model is called HCL. It was developed outside of CIE by Canadian researchers in 2005 and improves on existing models using a new similarity metric called DHCL.
See the wikipedia articles on color, color theory, and color vision for more details. This short document also describes the strengths and weaknesses of CIELAB.