I see, great explanation. If Google can't differentiate PBN with normal site then this technique will works for longer years unless they change their algorithm, like reducing site link's power if they don't have history in that niche, because so far if you build PBN with different niche than the old one, it still works, although it works best if the niche the same as the old one.There is nothing wrong with an old domain being bought and reused, many times not knowing it has been previously live. Unless the buyer is in our world they wouldn't think to check. A single site PBN is difficult to spot and penalise and a multiple site PBN built for a single business and not some off the shelf PBN is difficult to spot if there are no footprints and in amongst a natural link profile. Google ultimately picks up on the off the shelf PBNs because they really are 'public'.
From a business stand point,
Google adwords' revenue depends on the CPC bidding by real businesses. In highly competitive niches, bids can go over $50 per click.
Now if Google totally cracks down on pbn and all, INDIRECTLY the competition level will plunge to near zero. CPC probably drop from $50 to $0.50. (No pbn means less affiliate sites to compete against in serps.)
That is to say people most likely won't pay as much money to AdWords for advertisement.
In other words, Google loves competition.
Their aim is to manage the competition to maximize profit, and not crack down on this or that, they are not police.
And also, like @Nargil said, spending time and money to crack down pbn doesn't justify possible gain, if any at all.
It's really simple. A decent PBN isn't obviously a PBN.
If you create your pbn the right way the y should look like a legitimate site.
Could they wipe out most of the PBNs with a blink of an eye?
Yes.
Why they haven't done that?
Because they don't give a fuck. People who use PBNs are only a small percentage of people who would be willing to use Adwords otherwise. Spending money on cracking the PBNs down would not justify the possible gains. - That's my personal opinion.