It's because Google's constraint is number of engineer-attention-hours. This kind of thing probably just isn't a priority, when they have a billion other things they could work on.

Keep in mind that Google is not a product company, they are a data company. They work on the biggest most impactful items, and don't have a lot of time for one-offs.

Finally, it might not be clear at all that these sites should be removed from the index.

Google has vast resources and could easily get this stuff right, but chooses not to. Their protests about not having the capacity can get pretty comical. Note also that the claim about "working on the things that have the most impact" is tautological.

There may be cases where we drop low traffic sites from our index, which is separate from the search index. I don't how much of that is public information, so can't go into detail.

Well but my point is why can't Google recognize these phone-number-lookup sites as the chaff that they are? Nothing should rank lower than them in any search that would return their pages. Said another way: it should be harder for them to clog the top of the SERPs (of course "them" could imply any number of topics).