I bet you one of the reasons is the low cost and availability of Uber service.
Well I haven't checked the young person stats for France, but where I live there is no UBER, I have already mentioned the almost total lack of busses and if you want a taxi, you have to phone up and book it between 8am and 5pm. Outside those times you can walk or cycle. The nearest town of any size is 27km so it might take a while.
If you want to be taken seriously by the population at large, you have to be realistic.
I know plenty of people who have no driving license, so no need for a personal car.
Every Single One of them lives in a large city.
Every city non driver I have met, that has moved to the country, has taken their driving test and bought a car within 6 months of moving to the country.
As for the 17 to 24 group not buying cars? When I was 17, nobody could afford cars so they generally rode second hand motorcycles.
The changes since then, in the UK, have turned commuter bikes into incredibly expensive vehicles. The kids still can't afford cars and now can't afford bikes.
With insurance for a Nissan note, for a 17 year old, ranging from from £1,200 to £2,500, it has effectively closed the vehicle market to them until they are around 25.
Stats may look good on paper, but there is always a story behind them.