Its simple.

It is because most of us base our knowledge of the 100% scale on what we were accustomed to in grade school. It might slightly vary from place to place but it kinda went like this....

A = 100-90

B =  89-80

C = 79-70

D = 69-60

E/F= Anything Below that

The only acceptable grades to bring home (no matter what your family background is) is never lower than a C. A C is middling, and deemed as "Average" as far as grades go. No one sees an Essay with a "D" and say "OMG I am so satisfied with this GRADE!!!"

If youre parents were  strict about it, A's, B's, or no dice. No wonder gamers say 80 and above or no buy.

Cool.

We had a descriptors system at school... (excellent, great, satisfactory, needs improvement).

I did in early Elementary. After about 3rd Grade it changed to Letter Grades for me.

I agree with the mindset tho, A good game to me should be 70 or above, to give a game a score below that is a "failing" grade and gives the impression I should not buy it. No game that is at least decent should ever get a score lower than that. That is why I think it is horrible that Sites like Eurogamer score games so low, especially when everyone that reads it is percieving the score in THIS mindset.

Thats why when I see scores like 50s and below..... I become very sceptical of both game and reviewer. The only time you could recieve an F is when you basically didn't turn anything in and/or showed no effort. Its hard to deem any game completely unenjoyable.....like....at all. I can't think of a single game that I think that about and I have played plenty of games I did not like. 

Most do but many Reviewers seem to be basing their scale off the premise that "50" is middling Average. This would be ok in it's own right but in aggregated score, they severely bring down the total, when all who read it also percieve 70 as the ideal benchmark.

Serious question....

Who here buys games that they percieve as a 60-D Grade?

Interesting, In Vancouver its,
50-71 = C
72-86 = B
86-100 = A

But I consider anything below a 7 as a terrible game not cause of grade school, but cause I played games that are below 7s and they were terrible

70 or below is fine depending on context.
In School its not fine cause its not a C and C's are needed for Degrees as the saying goes.

In reviews, most of the time anything over 50% is generally considered perfectly fine, BUT in the video game world anything below 80% is considered a failure.
Why?
Simple, its because the ratings for video games seems to start at min 60/100. If 90% of the games on the market are rated 8/10 or higher, that rating systems doesn't mean anything. It looses all its merit.

Could you imagine if Rotten Tomatoes for movies had every single movie rated 80% or higher? Would be a joke. It's easy to see why no one respects this industry, its run by fanboys.

You're a little extreme lol but you kinda said what I wanted to say word for word.

I only purchase games that are 80 and above. It may seem a bit pretentious, but I find that whenever I have purchased anything lower, I have not enjoyed it very much.