That's because the movie rating system uses a 7/10 being average system. Reason being is that there are significantly more variations of bad than good. 7 is average, 8 is good, 9 is great, 10 is masterpiece. Meanwhile, 6 is niche, 5 is watchable, 4 is unwatchable, 3 is bad, 2 is horrible, and 1 is eldritch horror abomination. Not exactly word for word on the last bits, but the emphasis is on the subjective aspect of movie watching. A movie can be objectively below average, but not so bad as to be bad, and fit a specific niche audience, which would give it a 6. The average viewer will probably not like it, but if that type of movie is your thing, you'll likely consider it a 7 or an 8. Meanwhile 10s are just good films that even people who hate the genre can appreciate, while 1s are contenders for violating the Geneva convention on torture. 3s and 2s have the potential to become cult classics. 4 is probably the second worst rating you can get, since the film is bad enough to be considered unwatchable, but not bad enough to merit bad movie watching.