Mostly because it's so easy to add a category. Another thing is that one page typically belongs to several categories that can have different access mode. I think it's possible tom implement per-category access but there will be many interesting nuances.
That's correct... But on the other hand, categories are the main structuring tool in MediaWiki. And people want to apply rights based on their existing page structure. It's the problem of flat structuring - access rights are simpler and more evident in hierarchies.
The funniest thing I can think of is when I'm editing the page and add the category to which I don't have the view permissions. I click Save and my article dissapears from the view :) . Of course it's solvable, but I suspect there is a lot more similar cases that will require some additional efforts.
In both HaloACL and IntraACL category isn't like a mandate - to the contrary, it's like a grant - rights from different categories are added to each one.
The case in which the article disappears from your view after adjusting access rights will always persist (or the extension should check for the loss on each edit), and one needs to call admins in that case, because MediaWiki has no "article owner" idea (which is definitely good for encyclopedia, just like the flat structuring). Either the extension should implement "article owners"...