The fact that you'd be hard-pressed to find a review below 6.0 or something gives me the feeling that IGN is literally just the game industry's advertisement engine.
Those scores tend to become norms: only meaningful relative to other scores under the same system.
Also, user-scores tend to skew high as well. Look at IMDB scores, almost no scores < 5.
I think this is actually a common phenomena with
all rating systems where it's out of 10. People have a hard time when you say to sort things into 10 levels of quality, so they tend to hover in the 6-10 range, with 5 or less only reserved for truly abysmal fare, but if you give them a 5-star system instead, they're much more willing to grade across the full 5 ratings.
There's also a herd mentality. If you watched a show, and you think it's pretty bad, but still better than something others have rated 6.4 on the site, then you'll feel bad giving it a below-6.4 rating, regardless of what "actual" rating you think either show truly deserves. So you'll give it a 6.8 purely because it's better than that show others graded as 6.4.
So, for example, if you gave Star Trek Discovery 4/10 (techincally
just below-average if you assume 5/10 means "average"), people would ask "you really think discovery is worse that Corey in the House (5/10)?" And then you could point out that, no, you would (probably) give Corey in the House 1/10, if you had actually seen it ... which you won't. But at this point, people are just going to label you an asshole, not a discerning person of taste with a well-calibrated scoring system.
So: psychology. You should give someone a 5-star system instead of a 10-point system, but optionally allow half-stars.