"She's a god. Why make her a loli, kawaii-desu moeblob?"

Because that's how she's on the games? Maybe not so much on Mountain of Faith, but on hisoutensoku she literally jumps arounds like a frog and make emoticons face, even on SoPM and HM she looks very childish.
I mean, you're free to draw touhou characters in any way you want, that's one of the cool things of the fandom I guess, but this commentary is just silly, that's how she is, that's why is the most common depiction.

I always thought that Suwako might have been a fully grown and powerful goddess, but after losing her power she just turn out to how she is today. Maybe in the past she looked like Kanako.