Monitoring your kids media is often harder than you think. Most parents are not tech savvy enough to set up a good system. Just using parental control software won't work because kids figure out pretty quickly how to get around that stuff. Keeping computers in an open area only works if you actually monitor them while they are on there. Most parents don't want to have to block out 2 hours (

as if kids actually keep to the 2 hour recommended limit) of the day to sit and watch their kids go online.
Plus, it isn't just computers that need to be monitored now -- smart phones and other devices give access to the internet, too, so those have to be guarded appropriately. If your kid has a cell phone, that's lots of unsupervised opportunity to access stuff if you don't have it locked down appropriately.
You can still do every single thing right with your own electronic devices and let your kid hang out at a friend's house where their parents don't know how to do this or, worse, don't want to do it. My own daughter's first experience with porn was at age 9 when her best friend's house next door where the older teenage brother had left up screens of porn he had been viewing (which I suspect wasn't quite accidental, so she never played there again). So, then you've got to worry about restricting your kids to only hang out at your house and somehow not turn them into an outcast or a hermit.
TL;DR: Parents really can't monitor everything their kids do unless they want to go to a creepy level like keyloggers and never letting their kids out of their sight, so maybe society does have some responsibility to make sure that kids don't get exposed to stuff they shouldn't be seeing.