In nearly all countries any contract must be at "arms length". Otherwise you could do fancy things, for example regarding taxes. :-)
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.
Similarly, as I understand it In the US most noncompete contracts are actually unenforceable due to the concept of consideration[0]. Every noncompete I've signed offered me nothing of value in exchange for my agreement, they weren't part of the negotiation process and were simply presented with the health insurance and other forms on my first day with a implied "sign this or you don't have a job". However that wouldn't stop someone from calling your new employer and threatening to sue.
We may be talking about different things. Executives often negotiate a contract that contains a noncompete. That noncompete has clear consideration as it was negotiated as part of a total deal of employment, theoretically the employee is more highly compensated for this term of the contract. However rank and file employees are often not presented with the noncompete contract until their first day of work after salary negotiations have already taken place and the position accepted. Such an agreement probably isn't enforceable[0] as far as I can read[1] as it's hard to argue its part of the employment contract as the contract is signed after you are already employed and have reported for your first day of work[2]. If it was a employment contract it would've came attached to the offer letter.
[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...
IANAL. But, for example, in some jurisdictions you can write in the contract that all over time is already compensated with your salary. As long as the salary is significantly above market pay there won't be any issues. If you pay market rate and the employee is regularly pulling 60h work weeks, you are setting yourself up for big liability issues.
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.
https://en.wikipedia.org/wiki/Arm%27s_length_principle