Target audience is the biggest thing. Most is aimed at high schoolers. But at the same time it is something that people can relate with. Also adding in high school is considered the last time of happiness in Japan after that its college and work force. And getting into how terrible the work force is in Japan, so it is safe to say college most likely isn't much better.
Also keep in mind a vast majority of anime is based off Manga and Light Novels which has the target audience listed above. These demographics being Shounen, Senien and Shoujo. From my experience it doesn't matter the setting for manga in these demographics they ultimately treat it like the typical high school anime.
Josei is a demographic that aims for outside of high school and the characters actually behave their age. Here are some example.
Maomao, an apothecary's daughter, has been plucked from her peaceful life and sold to the lowest echelons of the imperial court. Now merely a maid, Maomao settles into her new mundane life and hides her extensive knowledge of medicine in order to avoid any unwanted attention. Not long after...
myanimelist.net
Having slept through all four of her alarms, the energetic Narumi Momose finds herself running late for her first day of work at a new office. As she races to catch her train, she makes a promise to herself that none of her coworkers will find out about her dark secret: that she is an otaku and...
myanimelist.net
Kind-hearted Himuro-kun is unfortunate: when in deep concentration, stressed, or flustered, he involuntarily creates blizzard conditions for himself and anyone in his vicinity. During one such incident on the first day of his new job, Himuro-kun encounters the beautiful Fuyutsuki-san, who helps...
myanimelist.net
Misery seems everlasting in Miyo Saimori's life. Born from an arranged marriage, she was quickly discarded after her mother's tragic death. Her father remarried, and her younger half-sister Kaya received all the affection, while Miyo was degraded to a lowly servant. Lacking the strength to fight...
myanimelist.net
Among the royal court, there is an individual feared as much as revered: the Raven Consort Ryuu Jusetsu. Residing in her jet-black palace and isolated from the emperor, she is the subject of countless rumors. Claimed to possess mystic abilities, she is said to take on any request—from finding...
myanimelist.net
Mappa already putting them into work when next season starts on a new project