finding a new job you will compete with people that have this 3 month notice so most of the companies plan hiring around it. also if you are unemployed in europe it will look really bad during your interviews.
now compare that to USA. you give 2 week notice, job market is very fluid so you can likely find a new job 2 week after stay there a few months helping the company and then use that time to apply to a new job in a better company.
Obviously the french model might be better for low earners/blue collars that do not have a job security. but for everyone else it is really hard to move to another company, managers know that too so they dont need to carrot you into a raise or a promotion as much as they do in US.