Since CLIs are used in scripts/automation there's much more pressure for them not to change. Essentially, they become an API so breaking changes are a big deal.

Since CLIs are used in scripts/automation there's much more pressure for them not to change. Essentially, they become an API so breaking changes are a big deal. GUIs, on the other hand, are very difficult to automate so they can't be used in that way even if people wanted. As an aside, I think a great foundation for a GUI application is a "server" API that the GUI interacts with. You can automate a lot of testing this way and you can give power users the ability to automate workflows. Web apps are already built this way, but if you're making a standalone app you even have the privilege of making a very chatty API, which makes testing even easier.