Prep schools are not for university, they're for "great schools" which is a separate system and pretty much unique to the country. Neither prep schools nor the bac are relevant to universities (outside of having the bac), universities are free-ish and anyone can attend (at least the first year, there's no entrance test)
Prep schools in France can be damn selective as well. And prep schools tend to have contracts with universities so drop out from prep schools can be admitted to universities (they might or might not have to pass an exam for that). Prep schools are somewhat equivalent to the first two years of university, with a bit of literature/philosophy and foreign language classes, but prep schools are much intense and exams are harder (disclaimer: I did prep school and a "Grande Ecole", and and also have an Msc from a University).