The issue is that there are some online activities which we all agree that children must be banned from no matter the cost to do so. Society has now started shifting in what actually falls in that group, but no one is arguing against the group entirely. There are very invasive solutions to this that have historically been accepted in the most agreed upon use cases, but not that society has started to shift on what falls into that category, we see a lot of friction as others don't want those solutions applied elsewhere.
Whitelisting was never an accepted solution for this worst category, and thus it will not be an accepted solution to any specific cases that society has started to move into that category (accepted to the portion of society moving it into that category, those who don't put it in that category will see such solutions are massive overreach).
Whitelisting is literally the only available technical solution. None of the others work and they can't realistically be made to. Anyone who tells you they do is either hopelessly non-technical, woefully misinformed, trying to sell you snake oil, or has an ulterior motive (surveillance).
I suppose something like the Great Firewall is also kind of sort of workable but that's government coordinated filtering so expansive that it begins to share more in common with whitelists than blacklists.
For every site that complies with age verification laws there will be uncountably many that don't. And that's before getting to file sharing networks. And then there's the dark net. Both of the latter are readily accessible outside of the Great Firewall.