The science of ultra pure silicon

They are legal because the actual wording of the agreement has to be as narrow as possible. So you would be barred from competing in the same industry, in a certain geographical region, etc. It all has to be very specific. Otherwise it just gets dismissed in a court.

If I sell you my self driving tech company and tomorrow I'm founding a new one, right next door, ideally even stealing all the former employees back, that's competing. If I move across the ocean or start building self sailing yachts that's probably good.

Basically the limitations can only go as far as needed to protect the legitimate interests of the parties. There's of course some grey area where those interests overlap but I think overall 5 years isn't necessarily exaggerated.

The thing most companies don't realize is that non competes are extremely hard to enforce. Longest I had was 2 years, and that was just as a sysadmin at a genetics company. 5 does seem excessive and likely unenforceable.