not for nothin anon but i don't think this is exclusive to joseimuke, it's more just any gacha game that is able to keep itself sustained for a long time either needs good characters to keep people engaged (and i'd say non-joseimuke mobage manage this, like fate and granblue) or a gameplay loop that is addictive enough to sustain it without (puzzle and dragon).

tbh one of the biggest joseimuke gacha games is famously lacking in story and good character writing, touken ranbu. that's not saying it's bad, there's a lot of media outside the core game to dive into if you want, but it's really not as simple as you make it sound.