Well, that too. While UV is nearly absent in sunlight at the sea level, there still is a lot of energy in infrared that is not visible to the human eyes. See here for examples: http://en.wikipedia.org/wiki/Infrared_photography
It's probably just a matter of trade-offs. The visible range does cover the peak energy of the spectrum of sunlight, and covering more of the infrared may just not be worth the cost of a more complicated eye.
It would be interesting to see some type of measure of the information content of adding more infrared in typical nature scenes.
