Because he had a trial and plead guilty. Most people don't get to have a trial, plead guilty, then reboot the entire process because they're worried how they pleaded originally would result in jail time.
The fact of the matter is that he drugged and raped a 13 year old. In 1977 he was allowed to plead that down to statutory rape, implying that the drugged 13 year old was a willing participant, just too young to give legal consent. He was too chickenshit to risk even a minute in jail, so he used his celebrity and wealth to flee the country.
If he was any random dude who pulled the same thing France would have extradited him as he walked off the plane. Because he makes snooty ass films they worship his feces and have shielded him for 37 years.
The U.S. should drop a drone on his house in France and be done with it. The entire French government is guilty of aiding and abetting a fugitive, so they're just asking for the collateral damage.