system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are movies all 1.5~2 hours long?

Who decided all movies (excluding short films) would be generally the same length? Why this length in particular?
gpt
It's an average at which a compromise occurs between holding the audience's interest and attention and the feeling of getting value for money paid. Any longer and the brain starts thinking about doing other things; any shorter and the brain starts thinking it's been ripped off because it paid for a certain "duration" of entertainment. A half-hour flick isn't interesting enough to invest in a trip to the movie theater. 

Kids movies are usually shorter than adult drama features because younger audiences have less of an attention span and have a much greater sense of the passing of time (which is why rainy afternoons lasted forever and killed us with boredom as toddlers but we barely remember them passing as adults). 

All of this can be overridden if you are really into something that immerses you like a videogame, and it's more about averages than about individual watchers.