Because Sakuya was enemy once, playable twice, and then part of three games (IaMP, PoFV, SWR) where there were many playables. Where's the seventh?
Aya, on the other hand, got HER OWN DAMN GAME.
The running joke is that since Aya's introduction, she's been in every game and even got HER OWN GAME in Shoot the Bullet, putting her as the only solo playable besides Reimu all the way back in HRtP. Thus, people say she sold her body to ZUN. It's a stupid joke, like most of Touhou fanon.