There are a lot of very smart people from all over the world putting everything they have into getting to the top of that site. It's not a trivial task for them.
A stop-gap solution would be to simply penalize anything with ads. Legitimate websites will still have enough "SEO juice" (for the lack of a better term) to offset the penalty, but brand new copycats with otherwise no inbound links to them from other legitimate websites (and no other business model beyond scammy ads) will never be able to rank high, essentially removing the incentive from setting up these sites in the first place.
Not to mention, these problems can be identified, prioritized and then tackled manually one-by-one. Stackoverflow copycats can be dealt with by downloading the SO data dump, parsing it and then severely penalizing any website where the bulk of the content matches the dump. Pinterest can be penalized by simply excluding it from image searches until they actually display the searched image without asking for login. So on and so forth.
You can't win them all, and you can't train an algorithm to win every time either, but you can manually observe what's happening and deal with the biggest problems.
The problem however is that the current status-quo is good for Google. They've got the monopoly on search (every other one is typically even worse when it comes to these issues), and spammy copycat websites happen to have Google ads or analytics so Google actually benefits from them too.
And when you get to that state, you've pretty much turned the entire web into content marketing for your ad sales business.
So - of course search quality suffers. Search quality is not the point.
They have a site that’s been polluting image search results for years, isn’t even trying to hide (unlike the SO copycats which could technically churn out an infinite amount of domains to work around bans) and they can’t even deal with that.
At the end of the day Google doesn't exist for the good of the public, they exist to return the highest possible investment for shareholders. It's literally the opposite of their job to improve search quality when competition doesn't force them to (because investors see that as a waste of money that could have been spent on further eroding people's privacy to enable higher and higher ad revenues.)