If you have just realised this, then welcome to the cynics club.
I'm not gonna rehash the stupid things reviewers say...i'm sure there is a blog or two who can do that. What I will try to do is explain the logic and reasons on why it is like it is.
Firstly, how hard is it to gain knowlage on a subject? Yes, you can wiki something and get the info on something, but as an old teacher of mine used to say 'you need to know what questions to ask'. To get the correct info, you need to have a good general knowlage on your topic. This means lots and lots of gaming, in our case. You need to know your history, have had played both the turkeys, the classics and the flash in the pans. Having turned your hand at a bit of modding and pissing around with the engines themeselves helps too - cause you'll know abit about how to make a game. All this equals alot of time spent.
Secondly, how long do you need to play a game to really be able to write a good (accurate) review? Well, it depends with genre. With sports - a couple of hours. A CoD clone? Enough time to quickly blast through story mode. But with a RPG - I would say anything from 20 hours for a KOTOR-style (linear style) to 50 hours+ for a sandbox in the ilk of Fallout 3/Oblivion. With a super-sandbox like Morrowind, possibily over 100 hours. That's a hell alot of gaming.
Reviewers are under pressure, and game reviewers are the same as any. They are often given 'baches' of stuff to review each week/month, with the demand that each review is x amount of words long. A batch could look like:
101 Minigolf World
CSI - Fatal Conspiracy
DJ Hero 2
Fable III
DeathSpank
Get Fit With Mel B
Now, off that list, there is only 1 game I'd actully want to play. The CSI is gonna require a hell of alot of time to play. Two of them I suspect would be the gaming eqivilant of pulling teeth. The others I'm indifferent to.
Now, there are rules to reviewing. Firstly, you will have to play just enough so it is not obvious to all that you are talking out of your arse. You are not allowed to be biased, even if you hate the genre, or the maker. You can't slate many games, unless it is voted crappy by all (the recent Leisure Suit Larry, for example). You have to keep the makers sweet, particulary if you work in print media and you are a reviewing a EA game or another big fish. (They pay you indirectly, through all the ads). Your editor will edit your reviews for 'tone', which usually means removing the most negitive comments. Last of all, you will be required to find a game to wet yourself over each month you may as well start wearing adult nappies.
Lastly, you won't allowed to write 'over your readers', so no big words or refrences to old games they will not have played. Your editor will live with the assumption that all games are still bought by 15-year old teenage boys who will have not ever played any game over three years old. And have the reading level of a 10-year old. Even worse, the editor is a jurno, not a gamer and the last game he played was Sonic 2 in 1994.
We need to remember, the reviewers are professionly interested in games. They must of had a flame of passion for them, at least once. But they are jaded. From playing batches of mediocre games, having a workload that is really too high. Sometimes being given games that are still in beta to review. Occasionly being given a few clips to watch of the game rather than the game itself. Having to discover a 'this years ultimate game' every month is tiring.
So they cut corners. Get a couple of screengrab pics, cut n' paste the blurb that the developer thoughtfully gave with the review copy (they DO exist - I have seen them!), pad the blurb with meaningless waffle and statements of the fuckin' obvious to the required length, then give it a nice score. Simples. As that idiotic meercat would say.
Me? I don't even really read them anymore, not when I'm looking for a new game to get. I don't even trust any game studio per se - with the possible exception of Paradox Interactive. What I do is wait a week or two, then go onto sites like this, and read what the game is really like, from people who usually care and have played it properly.