Why excalidraw does not use standard, open-source file format/extension?

[rebelC0der]

I was looking for an open-source, self-hosted MindMap tool, that uses some standard file formats.

I am wondering why most of the new and modern mindmap apps use custom file extensions like .excalidraw, and not .mm or even .json like some other mindmap apps?

Does anyone else have a concern, that they might build up a lot of work, using excalidraw, and one day, excalidraw becomes abandoned, and now you have no way of moving your work to another application? Why excalidraw has to use a custom file format?

Thanks.