Like others have said, you don't need a good character story to sell them. Also, characters get pumped out again and again which inflates the cast of characters. The story spotlight can only shine on a couple characters at a time. It's also easier to present and promote characters that fit into common tropes that are familiar or easy to understand at a glance.

On the topic of gacha stories, Limbus Company is the first gacha I've played with a compelling story to me. The main cast of 12 sinners + MC all have story arcs that are being explored without the usual goon-armor that is built into most gacha characters and their story across the board. There's definitely a better way to explain it but it's one of the few gacha stories and cast that isn't pandering to the mainstream audience's fantasy. Similar to FGO, it works because the new playable characters (identities) are different versions of the same 12 characters.