Some people don’t like very many characters. It depends on what they like to play.
For example, I’ve tried each character enough to know that I only like maybe 5-7 of them enough to play them regularly… =.=
I’m also the kind of person who needs a goal/something to work towards, and I’ll RAPIDLY lose interest in a game if there’s nothing else left to “achieve” - if all the characters I liked were maxed and my Command Rank was, too[1], I’d not be all that interested in playing anymore. There’s not enough content/maps at the moment and the gameplay, while very fun, get’s “samey” after a while (as any game does with enough time lol).
[1] Due to this ABSURD event I’m gonna be Command Rank maxed very soon ffs - I’ve gotten more than 25 levels in the past two days ALONE, a lot of em more than one per mission ffs! I’m almost to level 70 already…