As a former military police officer and currently in training for civilian police work, I'll try to shed a little light on the subject.
Certain laws are made, but as was stated, the ratio of officers on the road to people driving is a huge contributor to the "lack" of enforcement. Consider 300 million people in the US and under 700,000 cops.
There are state cops and their job is enforce all the little traffic laws you mentioned, at times, they are very busy with fatal accidents, having other persons pulled over etc. An officer can have a person pulled over and get passed by at least 5 people breaking every single law you mentioned.
Some city departments have traffic units to enforce these laws as well but the same situation comes up. When the traffic unit guys are not on shift, the patrol guys enforce the traffic laws, but remember they must also respond to calls for service and other such calls in the community.
Lets face it, people drive like retards, don't pay attention to the traffic books, and just do whatever they feel like they wanna do, regardless of legality. If it looks cool, they wanna do it.
We do what we can, as best we can, but we can't get every single one.