They only want downvotes in specific situations.

To help them tune things.

They don't generally want downvotes, because that breaks the way they want suggestability to work.

Its much harder for the algo to do its job when it tries to reverse engineer 'why did this person downvote this?'

But when it specifically wants to ask you that, it will.

Its job is to serve you an endless feed of 'close enough'. Not an endless feed of 'hey i actually like this'.