IGN being IGN.
While I agree with you that 7 should represent a 
good score, it rarely is in gaming.  In movies and TV shows, reviewers use the full rating scale.  That's not the case in gaming.
Let's look at a few past examples to see what review scores actually mean in gaming, and I'll take the total aggregate MC scores:
- Redfall = 56 (it's closer to 6/10 than a 5/10.  And it's an absolute dogshit of a game but 0 redeeming quality)
- Immortals of Aveum = 69 (pretty much 7/10.  A complete failure of a game)
- Concord = 62 (hot garbage)
- Suicide Squad = 60 (crap of a game)
- Biomutant = 69 (pretty much a 7/10 game as per scores, which should be Good, but it isn't).
These are only a few recent examples. 
All these games were considered bad and commercial failures.  Yet, according to the scoring charts by IGN and GS, none of them would qualify as a bad game (which starts with 4).
In gaming, games rarely get a score of 4 out of 10, even the absolute dogshit games like Redfall and Concord.  Nobody uses the full scale in video games, which makes 7/10 games quite mediocre.
Again, I don't agree with this.  They should use the full scale.  But this is how it has been.