I'm puzzled by the "This is the best game ever" and 10/10 flood of late.

[Quickdraw McGraw]

You're (OP) not understanding how a review system works.

A 10/10 (or 5/5, 100/100, etc) does not denote absolute perfection, or the absence of flaws. It merely judges the game on basis of its peers and predecessors.

In other words, the question a reviewer asks when contemplating a perfect score is not, "Can this game possibly, in any way, get better?", but, "Is this game the best experience available out there now, and will it influence future games?".

That being said, 10/10's (And scores near that benchmark) are thrown around far too much in the gaming press. Movies, TV shows, and even music will rarely get accumulative scores near 10, much less 10 itself.