The popularity of this title in the West mirrors Japan's trends. As the average age of the "otaku" demographic rises, the appeal of the "young male protagonist" wanes. Western players in their 30s and 40s often find it easier to identify with a tired, experienced "Uncle" than a high school boy. The fantasy of being "appointed" to a school serves as a potent escapist mechanism for an aging global workforce facing similar stresses as their Japanese counterparts.
The game acknowledges a cynical worldview where human relationships are transactional. The girls use the uncle for pleasure; the uncle uses the girls for his livelihood and lust. There is no pretension of "pure love" conquering all. It reflects a weary, mature outlook on life.
Since you cannot find the "Ojisan" official English release, here is how to access the experience legally or semi-legally.
Produced by studios including Lune Pictures and T-Rex , the series is categorized as mature animation.