Mostly for the same reason we only support our Docker based setup when you install Discourse. We can’t really help you when we don’t know how the development environmental was set up. There’s simply too much that could go wrong. Different Ruby version, wrong version of ImageMagick, missing tools used for image processing,…
I can only speak for myself, but I run all migrations in a Docker container. I use my development environmental only for tweaking an existing script or developing a new one.