Because people like reading about characters that they can identify with in settings that they are familiar with. That's true in any form of fiction across mediums, genres, and cultures, so it's not specifically a manga thing.
As mentioned above, most readers of shonen and shojo comics are high schoolers themselves, so high school students are characters they're going to identify with, who share their own problems or at least analogous problems. It's easier for them to put themselves in that character's shoes. And when the characters don't deal with problems that most high schoolers do, well, that's where wish fulfillment comes in. A high school student won't get as much wish fulfillment from reading about a yakuza don getting laid left, right, and center, as they would from a high school harem manga where a student their age has cute girls their age falling all over them, because it's easier for them to "become" the main character and vicariously experience their harem wishes through that kind of manga.
Meanwhile in magazines directed more towards businessmen, like Weekly Manga Goraku, for instance (which I myself read biweekly), you'll find manga that mainly have settings largely related to business or adult life in general, with problems and wish-fulfillment elements that those businessmen would be more likely to have. So you'd be more likely to see characters that cheat on their spouses than characters that wonder when their senpai will notice them, because that's more exciting for readers that have been married for a while and have a dull sex life.
These do exist and there are quite a few of them. But another problem compounding the issue is that in the West, your view of manga is more likely to be skewed due to the underrepresentation of certain genres and demographics. The common perception that "comics are for kids" keeps the majority of adults from reading comics, and among the minority that do, most stick to the comics and genres that they grew up with, rather than seeking out different genres that are more fitting to their stage of life. There is no market for the adult-oriented comics (and I don't mean hentai) in the West like there is in Japan, where there's not an insignificant number of businessmen who might pick up a manga magazine to read while riding the rush-hour train. So a financial manga like Emperor of Minami may be popular enough to run continuously since 1992 in Japan and be adapted to no less than 60 direct-to-video movies, but it would flop majorly if someone translated it into English. As a result, you see manga with high school settings as more represented than they actually are, because you're not seeing the full spectrum of manga available, only the tip of the iceberg that has been successfully marketed in the West. And that "tip" is mostly for high schoolers.
Now if you'll excuse me I have to go wash out my mouth for using the word "senpai" untranslated in an English post.
Edited by SystematicChaos, 16 July 2015 - 01:26 PM.