Agreed, recycling a comment: on reasons for it to be that way:
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1. Most of the dollar costs of making it all happen will be paid by the people who actually need/use the feature.
2. No toxic Orwellian panopticon.
3. Key enforcement falls into a realm non-technical parents can actually observe and act upon: What device is little Timmy holding?
4. Every site in the world will not need a monthly update to handle Elbonia's rite of manhood on the 17th lunar year to make it permitted to see bare ankles. Instead, parents of that region/religion can download their own damn plugin.
To expand on your #3, it also gives parents a way to have different policies on different devices for the same child. Perhaps absolutely no social media on their phone (which is always drawing them, and can be used in private when they're supposed to be doing something else), but allowing it on a desktop computer in an observable area (ie accountability).
The way the proposed legislation is made, once companies have cleared the hurdle of what the law requires, parents are then left up to the mercy of whatever the companies deem appropriate for their kids. Which isn't terribly surprising for regulatory capture legislation! But since it's branded with protecting kids and helping parents, we need to be shouting about all the ways it actually undermines those goals.