They are illegal in California. I have heard it said that has been a pro-competitive influence for companies here which is one reason for the dynamism of the tech sector.
No non-competes mean that companies have to pay you to keep you, otherwise you walk across the street. This creates the high-pay-for-tech thing that we've seen for the last few decades.
Well paid tech types can afford to take time off and create startups and pinch talent in lieu of equity