Ok, let me try to put it in a way that will hopefully make you guys understand.
PoFV Had 16 playable characters total. Which is more than 15. Each one with a different shoot type, and a different story dialogue. And all of them were made by a single man; Zun.
So, I don't see why bitching up so much about something that can easily be countered. It'd be time consuming, yes. But not impossible to make.
But seriously now, the Phantasmagorias are competitive games. Fighters in a sense. They have short and stories for each character so both the developer and the player won't need to put too much time in it and get it over with, so they can continue on the more important things. In a competitive game like Phantasmagoria the arcade mode it isn't about the story, but about training for competitive gaming online. Tekken, Soul Calibur, Mortal Kombat, the original Twinkle Star Sprites, many racing games and many more competitive kinds of games have limited dialogue and stories. SSB has zero dialogue and didn't even have a story until the latest installment, because it's all about the multiplayer. In the regular Touhou games a story needs to be told, and you're facing off against unique bosses which you can't use as practice for competitive play (since there is no multiplayer to begin with).
The main games are a lot more complex than the phantasmagorias. As such, it'd be a bore and a chore for both the developer and the gamer to make 15-30 characters for it with each their own story. Imagine having to play Super Mario, Kirby, Metroid, Donkey Kong or any similar game 15 times with only slightly different characters to get the whole story and/or achieve 100% completion. You'll die a lot, you have different difficulty levels, multiple endings, bonus stages and much more things that will force you to replay the game over and over just to get the complete story/100% completion. Doesn't sound very fun, does it? Yet the game will compell you to beat it all anyway, due to curiosity. In the end you wouldn't be playing the game for fun but just to reach the end of the while mess. You wouldn't enjoy it anymore by the time you replay the game for the umpteenth time with only slight changes.