All the forms in our *kihon* typically move from hanmi to hanmi.
They sometimes move through hito-e-mi between the hanmi, though we don't talk about that explicitly very often.
Interestingly, if I jiggled with the basic forms just a little, I could make them start in shizentai.
My thought is that the various kamae are just different pioneer's ways of teaching us their ideas about how aikido works. I.e., learning tools, not the end product.
As I read that again, I think "Duh", but maybe someone will find it useful.