I was very fortunate. I was single, and at the time I lived _very_ cheaply, and I had set aside enough money that I could survive almost a year without work. Before that, every job I had ever worked had that stupid clause, and I had swore I would never sign it again. I was only at this place because the money was really, really good. The work environment was the worst I've ever experienced. When they sprung that on me, it was the last straw.
They do this to you because they hold all the cards. I was just lucky that I didn't need them.
That's why it think these kinds of contracts should have very strong limitations since as it stands now an employer could potentially make you sign a contract that would effectively make you unable to earn a living and survive for years or own any intellectual property you may have developed in your own time for the purpose of say starting your own business.
They could cripple you if they wanted to.
To be honest there's not much difference between this an slavery and when signing is the difference between putting food on the table or starving everybody is a slave.
Of course you could always not sign but not everyone has the luxury to do that i would argue most people don't.
They do this to you because they hold all the cards. I was just lucky that I didn't need them.