Why do you use such high numbers when rating anime as opposed to people that rate in a more critical way, using all numbers (1~10, 5 would be the average).

First, see http://ask.fm/somekindofthing/answer/122156487342
I will give ratings from 1 to 5 sometimes, but it's very rare. Why? Because I don't want to watch "average" or "below average" shows. If I watch a show and think "this is worse than literally half of all anime ever made" then maybe I should be dropping it!
I believe that trying to rate entertainment in a way that forces everything into a perfectly sanitized bell curve is wrong, and an unfaithful representation of how people actually experience media. Entertainment isn't a continuous spectrum from 1 to 10, it's a weird and complex set of criteria that interact in different ways, with a rubric changing from one show to the next or even one day to the next. Entertainment is lumpy, not smooth!
*** It's also closer to logarithmic than linear! ***
Rating shows against an "average", putting aside the problems with such a word in the context of entertainment, would require me to watch almost every show. How else would I have a sense of what's average? Ideally I'd ALSO be equally disposed towards every genre, premise, animation style, and so on. I'd have to be an opinionless robot.
And that's certainly not reality! When I approach a new season (as I'm starting to do with Winter now) and choose 5-8 shows out of 40+, I am *by definition* pre-selecting the series I expect to like the most. I am already excluding anything I think I'd dislike, which is the stuff I'd give very low scores. Therefore, if I ever give a show less than a 6, that means I made a rather poor assumption about how much I'd like it, and if I'd known better, wouldn't have selected it in the first place.
Alright, so what if I were to take *only* that biased-by-design set of selected shows and force them all into the "5 as average" scale? Yeeesh, no, that would be terribly artificial. I'd be be raising the floor so high as to give some shows I enjoyed 2s and 3s. That would be like scoring the top 1% of income earners and declaring $300,000 USD/yr to be 1/10 abject poverty.because 10/10 means multibillionaires.
Objective rating only makes sense when all observers apply the same rubric. You can do it with mineral hardness or stellar luminosity, but not TV shows. If my data set is 5 and someone else's is 35, their 4 and my 4 would mean fundamentally incompatible things.
So people who 'rate critically" on a "half below, half above" scale either have much much much more time to watch anime than I do and can do 30-40+ a season with no filters applied, or (more likely) are just kidding themselves into feeling like they're "more objective" about their entertainment.
The one thing I think everyone *can* agree on is "the more you like stuff the closer it gets to 10", which is pretty much what drives my ratings!