ehm, isn't that because that question is generic in itself?
I mean: "why can't anyone write a proper video game review" ultimately comes down to "because either you like it or you don't", and movies, despite being credited as an art form much sooner than other mediums, have exactly the same problem.
"I like thing!"
"Why do you like thing?"
"I don't know!"
Because really, the point when someone can actually do a formal analysis of something they like, is also the point where they stop being able to like it on a casual basis. It just becomes math at that point. Which can be fun to do or listen to (mr Plinkett and Best of the Worst), but it's not what a casual audience in interested in hearing about when they opt to watch a review. So their main show has to remain casual and fun to watch to a significant degree, I think.
They both look very differently at movies 'as pro's' then when they do HitB, is what I'm getting at.
And really, does anyone even follow a specific critic these days?
They all point to rottentomatoes and the only thing that site does is change a review into 'yes or no' and put into an algorithm. And it's because of that structure that movies can rate 0%, which in ANY other review system is impossible. "no stars for you" just makes a reviewer sound petty, and we all know 'how to read' a review like that: "don't fucking go", whereas the other end is simply 'go if you want'.
This whole 'critic' business has bothered for a great while, so please excuse the 'random' aspect of this post.