Generally because although you can easily generate new versions, it is difficult to generate a new version that is different from the old version in a specific way, without also being different in 100 other ways that you didn't want.

It's like a revision control system where when you submit a commit that changes one line, which it faithfully records, it also records a change in dozens of other lines in the file. (Which leads you down the merry road of Stable Diffusion where you can "inpaint" that one line, but now it's not able to adjust the rest of things to accommodate that change because you told it not to...)