Weights aren’t source because the goal of having open source software is that you can know how the software you’re consuming works, and you can produce the final software (the executable) using the source yourself. When you only have weights, you are getting something like the executable. Sure you can tweak it, but you don’t have the things you need to reproduce it or to examine how it works and validate it for your purposes. As such open weights are not in the spirit of open source.