It's not so much that people are too dumb to do this or don't understand the danger, it's that doing this well is tedious and time-consuming and expensive. Setting up parental controls and monitoring them is a chore, you have to sign up for a bunch of apps you don't want and read outdated docs and troubleshoot random error messages and crap. Most of the good stuff costs money, all of the shitty stuff is free. Finding educational apps that aren't terrible requires research, whereas the worst garbage imaginable is heavily advertised.
So, I'm not saying you're wrong, but that this is a problem with incentives as opposed to a moral failing. I think there's a direct parallel to the rise of highly processed foods. The people 2-3 generations ago understood that junk food was unhealthy, and very few people fed their kids 100% soda and hot pockets, but junk food was so much cheaper and easier that people gradually allowed more and more into their diets, and now here we are with an adult obesity rate 3x that of 1960.
> "teach your child about the issues with being on a phone all day"
Edit to add, it seems most of your other responders are responding to this bit, which I agree is not realistic (children aren't rational to reliably refrain from something just because you can explain why it's bad) but I can see why you're frustrated that they're zeroing in on that part and not the part I responded to.