As a European from a poor-ish lower middle class background, I can tell you that even free university doesn't cut it economically.
The thing is, every year you stay at school is a year that you're a financial drain to your family and a year that you're not being an active money earner. For many, it's simply not affordable to be in their formative years until they are 22 (likely even longer if studies take more than 4 years, they in practice need a masters/phd in their field to succeed, or if they fail and have to retake some subjects).
At least in my country, the system is simply not made in a way that can support a person studying and working at the same time, for a variety of reasons (lack of part time, weekend or flexible jobs, amount of work that one has to do at home, uni schedules that don't account for job-study balance, etc).
On top of that, there's the apparently less important, but in practice enormous psychological pressure of not having an actual adult paycheck until well in your adult years.
A life where your friends that went through a fastest route to the job market are living with their SO, spending money in life experiences and traveling while you're 24 and still have to ask your parents for money to have a beer, eat at mcdonalds or go to the movies (knowing they can't afford much on top of that), or where you can't take a girl to your house because you still live with your parents... it's basically assuming that you'll have to live like a teenager until you're almost 30.