Think of it similar to alcohol laws. E.g. in the US you don't just want kids to be unable to buy alcohol when their parent is present, you want the point of access to participate in that restriction or it won't work in practice because getting 100% of kids to have good judgement all of the time (or watching them constantly for their whole life) is not realistic. At the same time, many states still have laws allowing alcohol consumption by minors in the presence of their parent because it's really hard to get everyone to agree on a universal binary cutoff with no exception.
In practice, the law does help greatly in spite of not being a mathematical proof the minor will no longer get alcohol because of "parenting well" alone and almost all parents are fine with the restrictions, even in places without the flexibility, because they've come to see and agree with the level of harm over the years. Ie. there is a point enough parents agree strongly enough that the common good of children is accepted over the rights of a parent to decide their child's welfare - it's just usually a hogh bar (e.g. how far punishment can go before it turns into abuse, as another example).
In practice, the law does help greatly in spite of not being a mathematical proof the minor will no longer get alcohol because of "parenting well" alone and almost all parents are fine with the restrictions, even in places without the flexibility, because they've come to see and agree with the level of harm over the years. Ie. there is a point enough parents agree strongly enough that the common good of children is accepted over the rights of a parent to decide their child's welfare - it's just usually a hogh bar (e.g. how far punishment can go before it turns into abuse, as another example).