Definitely not questioning your opinion and review of the game (i quite enjoyed it but that's my opinion). However, i do not believe that most people would see 7.5/10 and think "that's a high score."

One variable here is medium bias. Different mediums of entertainment have different "average" scores. It seems (i think) like things that take more time to enjoy (video games, TV) have higher baseline scores that are considered average than say a movie. (As an example, metacritic has different score tiers for movies than it does for video games, for their video games, anything 60-79% or 6.0-7.9/10 is mixed or average, but this score is 40-59% for movies).

I would further say that, across the board, a score of 5/10 would almost certainly be considered well below average for a game (or even a restaurant for that matter). I would be willing to bet that average for a video game would fall into the 7-7.5 range for most people. There are probably other biases at play as well, many of which i believe are impacting the scores in the first place (for example, the way people rate movies may be even more critical, and then perhaps there are so many highly rated games that it brings up the average there).

Something that i know i consider is that video games can absorb 10s, 100s, and even 1000s of hours of time, and i wouldn't want to sink that kind of commitment into a 5/10 game, which probably got such a score due to a combination of bugs, bad story, and poor mechanics. Plus, the lowest ever rated games are 1-2/10, and you'll be hard pressed to find any game rated below that (which further drives up the average).

I know that If i saw a game with aggregate reviews of even 7.5, i would think it had major problems and may not be worth my time investment. 7.5 is almost certainly not a "high" score, though i can appreciate you being very deliberately mathematical with your scoring.