That's the question that's been lurking for me behind this entire thread. He was visual architect of the Marvel style, and the sketches he was given were being done by people doing their level best to ape that. Based on his general mood in the decade following, I'd say it was yet another insult that he stewed on.

I love Kirby and Janson, but that was a bad combination in my opinion; the only one of those Defenders covers I don't like.  There is no clear focal point, and the character posing is very odd.  What is Valkyrie doing?

In this case I think the problem is Janson.  They have completely different styles, and his attempt to mimic Kirby's style in the cover sketch doesn't work.  The Milgrom covers look a lot more like something Kirby would draw.

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As mentioned upthread (Jason on page 4), apparently Kirby WANTED to be "art robot" when he was talking to Roy Thomas about returning to FANTASTIC FOUR.  Maybe he didn't want to think about the work too much or maybe he was all plotted out on FF.  Maybe he just didn't want to think about these covers either.

As I've said, I loved his writer/artist stint on CAPTAIN AMERICA--I think it's his best solo work!  I recently read his complete ETERNALS for the first time, and that seemed more than just a job to him.  I don't know how invested he was on BLACK PANTHER, DEVIL DINOSAUR, 2001, or MACHINE MAN, but he seemed to be doing exactly what he wanted to be doing on CAP and ETERNALS at least.

Maybe he was saving his creativity for those two and enjoyed being just "art robot" on (not thinking about) the random covers.

But Kirby did get to draw Magneto for Captain America Annual 4, and even referenced the X-Men in one panel. 

So would we, Cap ... so would we!!

Kirby did not feel "plotted out" on FF.  According to Roy Thomas, the reason Kirby was only willing to be art robot in a prospective collaborative venture is that he was concerned that otherwise he would be asked to do too much of the work for too little of the credit/pay.  Roy attempted to address this with discussion of paying him for plotting (something he had not received in the 60s) as well as the idea of giving him top billing.  But Kirby's prior experiences with the Marvel Method in the 60s had left him pretty much unwilling to ever work that way again.  It's notable that every single post-1970 instance in which Kirby worked with another writer (with one exception) were done full script.  He really hated the Marvel Method.  The one exception of course was the Silver Surfer graphic novel with Stan.  Stan had not written a full script in decades, and wasn't going to start then.  

Alcala inked Kirby on the Destroyer Duck series.

I think it's pretty understandable that at that point, with little mind-reading, he just wanted to work but not give them any more than that for free. (And he couldn't have imagined at the time what happened in these past 10-15 years, when the real money came along).

The one and only good thing they did, in my books.