They are absolutely enforceable.
A court might not enforce them, because, depending on your state, they may be of questionable validity.
But people who don't know that will believe they are enforced.
People who aren't willing to risk going to court will have to act as if they are enforced.
People who can't afford to go to court will have to act like they are enforced.
*Additionally, I have seen these contracts enforced outside the court system as well.*
I had a friend who was fired from a job he went to, because his former employer called his future employer, and threatened a lawsuit / threatened to drop mutual business because the future employer helped the employee break the non-compete agreement.
The past employer may not have had any case (except for the lost business), but the future employer didn't want to deal with legal trouble, and it was easier to just find a reason to fire the employee.
How sure are you that you won't be fired at your new job, because a conversation like that took place behind closed doors?
How sure are you that McDonalds would be willing to foot a legal bill to defend their new minimum wage employee, if Burger King threatened legal action?
Non-compete abuse needs legal reform.