75% is a C which is average for school grades, and a 7/10 is widely considered an average score for things like movies. 70-75F being the average room temp is pretty intuitive when used alongside other common scales.
Well, if the average American views 7/10 as the average on a 1-10 scale, it really just proves that their view is skewed, objectively incorrect, and cannot be used as a useful measure in this. Being wrong in multiple (and completely unrelated) instances does not make any of them correct.
I mean I never said it wasn't stupid, just that it's intuitive for Americans. 5/10 should be average.
But I'm not even sure that's purely an American thing. Go to a rating website like IMDB or MyAnimeList, 7/10 is considered average. I'm genuinely surprised that you're surprised by this.