Because otherwise people could crash and severely injure themselves and others. I think that this would be the wisest course of action for the people making new roads. Why not just prevent most of the traffic- caused crashes and injuries. It would be better for everyone involved if we do this right away!
Some people are such jerks and speed through yellow lights when people have to wait to turn. Plus, its not fair to deny someone the right to turn, just because some speedy jerk does not want to wait. Its not always safe either. Without having to worry about finding the right time to turn. Since it is not always easy to tell when someone is about to stop, or if you have enough time to turn, since people dont always obey speed limits
How many people are hit by a car because they didn't see the crosswalk or person? Not many. Most pedestrians deaths are caused by jaywalkers or drivers not being attentive or being intoxicated. These do not have to do with whether they can see the crosswalk or not. The money spent would be substantial and would take much time to install the lights. And if batteries need to be changed, or if the lights are strung under the road to be plugged in, that's billions of dollars spent nationwide. We are trillion dollars in debt and we need to start doing something about it.