A ten point scale places the number 5 in the middle, meaning the average. Anything above the average means it is, appropriately, above average, while anything below it is below average.
1 - Unwatchable Filth
2 - Absolutely Terrible
3 - Bad
4 - Below Average
5 - Average
6 - Above Average
7 - Good
8 - Great
9 - Amazing
10 - Perfect
Since perfection is realistically impossible, it generally forms a nine point scale.
Part of the problem comes from hype distorting people's views of what they actually watched (mostly common among children and teens, admittedly), and another part of the problem comes from people liking to label all things they don't like as equally bad, effectively making it a scale of 5 - 10.
What is the point of a rating system where half the spectrum isn't even used? Just to make people feel better, because the things they like have a bigger number? I thought people hated IGN and such for those exact reasons?
My problem with MAL is the exact opposite; that people aren't critical enough and have a tendency to give 10/10, 9/10, or 8/10 scores to anything they like, regardless of how good it actually is from a scrutinized viewpoint.
Monster Musume has a 7.50 rating currently, and it's a bog standard by-the-books harem series that just happens to have monster girls in it, does that really justify a 7.5/10? (Keeping in mind that 5 means average to any legitimate critical mind, an absolute medium, mediocre.)
Even in terms of harems it is nothing special, nothing that justifies a rating like that, but many people have the opinion that anything they enjoyed MUST be amazing or it would mean they don't have good taste, and that's impossible, right?
Considering that it's 7.5 ranking puts it at number 1423 on the top anime list I think its less of people on MAL actually thinking its good and more of that they just don't agree with your ranking system.
I feel that a lot of public ranking systems tend to end up on the higher end of the scale because thats what we're taught in school. In school a 7/10 is indeed quite average while a 5/10 is a complete failure.
I wouldn't look to the school system as an example of doing things well in most cases.