Q:

Why do "identify all of these characters" questions get downvoted? 

Many of the "name all of these characters" questions get several downvotes. 

Who are the 378 mutants in this image? 
Can you identify each character and spacecraft in this Star Trek TNG 30th anniversary poster? 
Who are the characters on this Legend of Zelda poster? 
Who are all these superheros in Avengers: Endgame poster? 
Can we identify all the characters in this Universe 113 poster? 

There are similar questions, all asking people to name all of the characters or objects within a poster.  Most of the ones I checked have quite a few downvotes. 
None of these are my questions, so I don't have any skin in the game.  I am just curious why the number of downvotes for these tends to be higher than average. 

A:

To be honest unless I really like the image I tend to stay largely away from these questions and as Rand stated in the comments most of these questions are largely upvoted, it's just they also attract downvotes.  I can think of a few reasons though:

They can be quite boring.  Some are "here's an image of some well known characters, who are they all".  It's not really a challenge for most people and quite boring. 
To tie in with the above point the questions rarely show any research effort which is one of the reasons to downvote.  The question is generally of the form "here's an image who/what is in it".  Even when here's some of the most highly recognisable characters in there the OPs rarely mention them. 
Asking to identify a few hundred people/things is bordering on "Too Broad".  Whilst we've decided as a community to keep them and not close them some will still downvote to voice their disagreement with them. 

To tie it together, there are just some people who don't like these questions and that's fine.  Their reasons can vary wildly from person to person and question to question and I have by no means covered them here. 
At the end of it all just look at the score of these posts which is almost always overwhelmingly positive.  It doesn't matter about the odd downvote here and there when the majority are upvoting them. 

A:

I agree with most of TheLethalCarrot's points, but my main quibble with character-identification posts of this sort is of a somewhat less community-based feeling, namely that for the cases with loads of entries, it's practically impossible for one person to provide an answer (although exceptions exist), so you wind up with a community wiki answer and no one gets any reward for their hard work, while the person asking typically reaps a bounty from all of the activity pushing the "popularity" of the post up with high views and a lot of action. 
I know... Stack Overflow isn't a popularity contest, but part of the system is that people who are active, and helpful, are rewarded.