This assumes that the images on disk are not going to change between subsequent invocations of the image picker, which is too strong of an assumption in the general case. Technically, it might even be possible (although highly unlikely) that an image could change between when it is picked and when App Inventor copies it to App Inventor's working directory. This behavior ensures that for the average App Inventor user once they've picked an image it will remain and be available for them to manipulate.
This is an unfortunate side effect of Google's rule changes. The goal is that eventually people could write to the shared storage so that the picker would be able to use those images.