It seems like we're all agreed that unmonitored, free-for-all access for kids onto computers or phones is risky and potentially unhealthy. Where I am confused

It seems like we're all agreed that unmonitored, free-for-all access for kids onto computers or phones is risky and potentially unhealthy. Where I am confused is - what might make that hard to govern? I fully appreciate that the tools 20-30 years ago were not where they are today. But now?