Here in France we can have them as well but there is a counterpart: if I want to go to a company which is a competitor, my current employer can say no but they have to pay me for that for the time of the non-compete (something like 75% of the salary). This is the law and they have no choice (short of not enforcing it)
French here. The exact amount is negotiated (aka decided by the employer) and written in the contract. I had multiple work contracts where the indemnity would have been 30% of my annual salary, never more (which is actually pretty bad because you still have to find a job without breaching the non-complete).
Non-compete clauses must also be limited in scope (geography and profession) and must not stop employees to live from their trade. As a result companies rarely exercise non-compete clauses since they are regularly thrown away by courts ("conseil des prud'hommes" in french).
Still they are part of the so called "standard work countract". I imagine most companies just get their base contract copy-pasted from the same template.
The amount also depends on the employee (some employees can actually negotiate if they have the upper hand, others as you mention get what is in the contract). On top of that there may be provisions in the collective agreement (convention collective). An example is the collective agreement for metallurgy (CC Métallurgie - this is a French thing and the name is historical) where the lowest compensation is 50% of the salary computed over 12 months.
That is a thing in other countries as well, and is usually what makes the non-compete enforceable. It's the idea of "consideration" -- in that the other party is getting paid in exchange for not working in that industry.
You see it in Oil & Gas a lot. Take 2 years off w/ a 80% pay, teach at a local community college or something for a minute, then back to the industry, etc. etc.
Here in France we can have them as well but there is a counterpart: if I want to go to a company which is a competitor, my current employer can say no but they have to pay me for that for the time of the non-compete (something like 75% of the salary). This is the law and they have no choice (short of not enforcing it)