It is likely that most people do not consult a lawyer when signing a long legal contract (or clicking through one on a web site), so they end up signing away rights that they later find that retaining would have been better for them. Even those who would want to consult a lawyer may not have the money to pay for one.
But then, most lower level employees have little negotiating power compared to an employer, since they probably need the job more than the employer needs them.
Both of the above are presumably why some states have laws against or limiting some types of agreements (like limiting or prohibiting non-compete agreements, or limiting the scope of invention assignment agreements) to protect the general public in interactions with more powerful entities like employers (who may have armies of lawyers at their disposal).