Because there is no way to understand why you're accepted or not in a given training program.
The system looks like it mimics what exists (or at least, what existed 25 years ago) in "Grande Ecoles" (Elite Engineering schools like Polytechnique, Centrales, Normale Sup), etc... - you make "wishes" to enter that or this program, and you're offered a spot in some of them and denied the spot in others.
However, the huge difference is that the algo in Grande Ecoles is pretty clear: there is an entrance exam that serves as a competition. The exam is precise enough that you're going to be graded relatively evenly no matter where you live, and the examinators don't know you. In the end, you get a ranking.
Then, depending on how selective they are, each training program is basically going to offer a spot to the first N people who wanted to join, ranked by the score at the exam. Very harsh if you miss the training of your dreams for half a points, but pretty simple to understand.
Parcoursup, on the other hand, gives zero information about why you're accepted or not. It's very different from receiving a letter from Yale telling you "sorry, your grades are not good enough, your applying essay did not mention diversity / freedom / whatever, etc...")
People fill the blanks with rumors ("they don't take people from this city", "they don't take people with that last name", etc...), heuristics ("it's better to be the first of a bad class in a countryside high school than being the fourth in a very good class in the cities"), conspiracy theory ("someone hacked the system to remove my kids names", etc...)
Kids don't have a point of comparison, parents usually only face the kafkaesque system once, so it's hard to build a reformer base, and the system changes every two years anyway. (Which is the glimmer of hope: it might converge to something halfway decent in the long run.)
It's entirely possible the initial plan was completely different from the current implementation - or maybe there is a missing piece that never came to exists (replace the baccalaureate with a ranking exam ?)
Political opponents would tell you that it's on purpose to limit people entering university to lower the costs (I mean, we have pensionners to pay at home and abroad.) ; or that life is simpler if you're rich enough to enter private training program (where the algo for entry is "cash or check ?")
The one good things is, that, as far as I know, once you've gone through the hurdles, most universities, engineering schools, etc... are still very cheap in France (to the level that USA would call it "free" in comparison). Even the most elite ones.
The former system had drawbacks: you would basically register wherever your want to study whatever your want ; the first few classes of the university would be crowded beyond reason, and then half the student left when they discovered they did not really want to study psychology or history of art for a living, but, hey, that's youth ;)