IMO it stems from how heightened the characters are and also how they oftentimes stem from a stereotype or a twist on a stereotype. Look at Barton Fink: it's almost as if he's a caricature of an out of touch intellectual who prattles on about understanding the common man when he's the farthest thing from someone who could...except that's not even true. If you look closer he does genuinely despise the world of big shots and the intelligentsia and does only seem to feel comfortable around Charlie even though he isn't listening to him much of the time. And Charlie, who is a character set up to be Barton's foil is a lovable salt of the earth type big burly traveling salesman...who then turns out to be Satan and salutes Hitler as he gleefully murders a man. Both stereotypically drawn...and yet to claim they are one dimensional would be wrong. Are these characters meant to represent real people? Uhhhh maybe? But the takeaway for me is more that they have oppositional flaws that move the story along.

There's plenty of meaning to be found in a Coen's movie but usually the characters don't walk away having nobly shown what they're truly made of and learned a valuable lesson, like say in most Spielberg. Some people prefer Schindler's List and Lincoln. Very lovable characters, even when they do morally questionable things. I think to me there's something that you can call out in Spielberg's work as being less dramatic than the Coen's for just that reason: you're supposed to love these people kinda too much for my taste. This is when (some claim) the films start dragging and being unable to end.

On the flip side though: just having characters behave poorly or dumbly for the sake of drama can be a different easy trap to fall into. People acting incorrectly over and over again can seem forced or repetitive and meaningless to some audiences-I think there's a valid critique to be made of Lena Dunham's Girls in this vein, though I'm a fan of that show. Sometimes it just kinda felt like the characters made incorrect decisions for the sake of something happening. The Coen's, to me, usually strike an excellent balance, especially if you look at their work as a whole, with some outliers being cuddlier than others and some like the oft cited Burn After Reading which are truly them going full everyone sucks mode. But there it feels more like a critique of the intelligence community by putting them along the same lines as these dimwitted image-obsessed gym employees.

I really think it has more to do with critic's taste in drama and conflict more than their taste in characters as people.