My best guess is it's because they finally have a real competitor in ChatGPT.
> The real problem is the financial incentives that make this kind of spamming profitable in the first place.
Yeah, but the financial incentives exist on both ends. There's a gross symbiotic relationship between Google and SEO spammers, because Google also owns the ad network the spammers put on their page. If Google puts ad-laden SEO blogspam as the top result and a user clicks it, the user sees a bunch of ads from Google. Everyone wins: Google, the SEO spammers, and advertisers. Well, everyone except the user, but who cares about them?
My guess/hope is that ChatGPT has made someone who actually cares about the quality of search results actually step in and say things have gone too far.