In other fields the obscurity is a side effect. In the humanities it’s the point, to try to make the sky-castle ideas sound more Important and Scientific than they actually are. A big part of postmodern theory is the notion that there basically is no truth, just texts with varying degrees of authority. The field unironically describes itself.

No, it really isn't, and virtually no one who earnestly engages with the thinkers in question would come away thinking that.

I “earnestly engaged” with it by doing a humanities degree 20 years ago when this stuff was taking root.

The only field that seems resistant to being overwhelmed by it is history, possibly because the scholarship must be grounded in original sources.

Yes, there are obviously people in academia that are full of crap: other people in academia -also- think they are full of crap. The problem for outsiders is that they're largely incapable of discerning between the earnest deep thinkers and the self-promoting charlatans. Charlatans are always louder, so without stopping to understand the topic at hand, you're almost certainly giving the most of your attention to those who least deserve it.

It seems pretty contemptuous to suggest that people in the fields you don’t really know about are trying to obscure some secret bad opinion so that it can become accepted. Maybe these people are actually trying to express something complex and nuanced?

I think there are a lot of issues with academic writing being obscure but I don’t think these are unique to the humanities. Indeed I think you’ll probably find many people in humanities fields are better writers than those in the academic areas you may be more familiar with.