why those same members of these curation groups constantly vote for extractors

So the way I see it, is that many of the 'curators' that 'work' for curation groups also only really care about getting some money out of it and operate in the most efficient way, similar as shitposters who just pump out content that without any potential earnings never would be there. So they look at specific communities or challenges and find some users who systematically make posts that look ok or meet the required number of words to be considered 'quality'. They eventually get to a list of users, completely neglecting KE and overall behavior and just upvote those all the time, as it makes it a very efficient way to 'curate' and get some earnings here.

To their defense (if this theory is remotely right), you can't really blame them because it's super hard and not fun to really plow through everything here in the hope to find some actual quality that is underrewarded.

I doubt it’s a small part of the problem, because DHF accounts for 10% of inflation and the reward pool accounts for 65%.