Certainly one can blame parents who are willfully ignorant, ignore advice, think their kids are invincible, etc.
But it also seems that (around here, at least) there is an equally large group of parents who are trying to be conscientious, but who are bewildered by the complexity here. There are infant seats and convertible seats and booster seats; seatbelt installations and LATCH; ALR belts and non-ALR belts; the intricacies of locking clips, chest clips, buckle tongues, seat bights, etc.
Each vehicle is different and each seat has its own idiosyncracies -- our Radian, for example has a "Safe Stop" that MUST be used in some situations but MUST NOT be used in others. It's not uncommon to require a rolled-up towel to achieve the proper angle or for a particular seat to just be incompatible with a vehicle for some reason.
Yes, it helps to read the manual, but I've found errors in the manuals, too. In looking up lap-belt installation instructions for our Britax Roundabout, the manual includes a line about pulling on the shoulder portion of the belt. This is a lap-only belt -- what shoulder portion? And most of the manuals bury the important info about locking clips in a separate section.
It's also difficult to keep up with changing recommendations. A lot of people who dutifully research the exact region-of-origin in their kids' organic green beans are still only vaguely aware of the new push to keep kids RF until 2.
Professional communities who are in a position to help (police, pediatricians, transportation departments, and others) are still not giving this the attention it deserves. For example, our pediatrician's office provides a little info sheet at each check-up, always noting the need for a carseat along with vaccinations, developmental milestones, etc. But they certainly don't view it as within their duties to go and check the installation.
Likewise, in our city, the police department has severely cut back on installation clinics and I've never heard of anyone being pulled over for improper CRS use.
At the federal level, NHTSA notes the need for carseats, but their installation videos give the impression that you just need to click the LATCH connectors in. To be useful and realistic, they need to actually show the parent pushing down with all of their might on the seat as they tighten.
Our kids log more miles by air than by car, and it's shocking how many flight attendants are completely misinformed about CRS use -- and they are professionals. Imagine how confused the parents must be.
I'm not making excuses for people, just pointing out that it's everyone's responsibility to make things better. Automakers, for their part, could help by developing built-in carseats that would be appropriate to the vehicle and eliminate installation issues, but for whatever reason (cost? liability?) they have not done so.