Because we are spending a small fortune on traffic lights, their installation and of course subsequent maintenance.
Most traffic light installations seem to have far too many lights. For example you will often find two poles a few yards apart on the left at a junction. I suspect this is the result of successful lobbying over the years by the manufacturers who are making a fortune at the public expense.
They don't do this in other countries. France is a good example. Usually a single pole with a small repeater low down so the dirver can see it when he or she is close.
Less traffic lights also means more room on the pavements for pedestrians.