Maybe the next employer doesn't have a full time lawyer or doesn't think it's worth the hassle. Or they don't want to hire someone who is going to have to quit

Maybe the next employer doesn't have a full time lawyer or doesn't think it's worth the hassle. Or they don't want to hire someone who is going to have to quit in a month because they decide the contract is enforceable. For what it's worth, I have seen it play out in the exact opposite way. I was working at a company that took great pride in having the best engineering talent and also happened to have a very good (very expensive) in-house counsel. We hired a crop of engineers away from another company and subsequently got a nastygram from the previous company about hiring the guys. If anything, it convinced the management that they made the right decision to hire the guys, and they put the in-house counsel to work shredding the previous company's legal arguments. It was pretty fun to watch.