This is actually a reason why you can not, for example, enforce something that is given freely. E.g. I give you a contract where I promise you to give you my house for free. If I change later my mind, in many jurisdictions you could not force me to write over my house since there is not "arms length". It is something for nothing.
[0]"Performance of existing duties is not good consideration" - https://en.m.wikipedia.org/wiki/Consideration
[1]"The very basic requirements are that the non-compete must (1) be in writing; (2) be part of an employment contract; (3) be based on valuable consideration; (4) be reasonable in scope of time and of territory; (5) not be against public policy." - https://lincolnderr.com/is-my-non-compete-enforceable/
[2]”One of the most common reasons that courts refuse to enforce Non-Competes is that employers make the mistake of obtaining the agreement from an already-hired employee without providing the employee with anything of value in return. Generally, such agreements are unenforceable because the employee did not receive any additional “consideration.”" - https://macelree.com/the-top-10-mistakes-with-non-competitio...
Sure, for some people, 18 months off at half pay or so might seem like a pretty sweet deal. It certainly isn't for everyone.