In front of you stands two cartons of ice cream. One is one pint. One is two pints. They are the same price. They are available in the store at the same time. They are the same price. Which one do you purchase?
Congratulations, now you know why “5/10 is 'Average'” is a stupid concept.
Time and money are fixed concepts to everyone. Nobody has as much time or money as they'd like, and that's why the concept of a “full spectrum” review system doesn't exist. There's no room for crap. Why would you waste your precious game time with something “average” when you could walk into a game store and grab, god, two dozen amazing titles without having to blink?
“Average” doesn't happen because it has been bred out of existence.
Something we would consider “average” by modern game standards is – often – thrown away before it meets the public. Either that, or it turns into, say, Mindjack: Shuttled out the door by a publisher too drunk on their own hubris to know what makes a good game anymore. (Oops, off-topic!)
Asking why there isn't proper use of the full scale of game scores is, honestly, stupid. Let's look at it from a different direction. Why not review scores for...surgeons? Who wants to go to the surgeon who's rolling a 4/10, just because he's “quirky” and “indie”? Nobody. Because you're not a fucking moron.
History lesson: A long, long time ago; back in the 8- and 16-bit eras, review scores were honestly a useful metric. The barrier to entry in making (mass-market) games was much lower than it is now. The entire field of game design was immature compared to how it exists these days. It was entirely possible to put a product through a complete design cycle and release it without a single person noticing it was crap.
Look at game development these days: Pretty much every second of the design and creation process is collated and broadcast all over the world via blogs. It's impossible to release a screenshot without it being dissected, scrutinized, and gone over with a fine-toothed comb by a public hungry to tear something apart, due to their misplaced hatred of the people who create the things they claim to “love”. If something is crap, you're going to know weeks if not months if not years in advance of an actual product release.
A truly wretched, worthless release in a field where you will spend millions of dollars producing a single product that – if you're lucky – will have a shelf-life of maybe two to three months before it's reduced to the twenty-dollar bin at Walmart is a rare thing indeed. Years of effort, thousands upon thousands of man-hours...in the bin of “meh” junk stuck in an aisleway where a manager hopes to God someone will take it off his overstocked hands.
Why isn't the full review score spectrum “apparently” utilized? Because if you're on the low-end of that scale, chances are you won't survive to be a repeat offender. Great studios make lots of great games. Three Gears of War titles. Over a half-dozen Ratchet & Clank titles. The fifth Elder Scrolls title just recently released. How's that Too Human trilogy going, guys? Oh, right.
P.S.: 5/10 is only “average” if there's an overall equal distribution of values along the given range. At least learn math before you try to use it to prove a point. Damn.