That's presumably because of exactly the problem mentioned in the article. Customers of the other software are forbidden from using those files with other software, which could potentially include the use of a tool to convert that to a better format, so the competing software just made up a different format.
I am ill informed about the actual industry landscape, but, Spine is listed elsewhere in this topic.
I wonder if it would've been better to ask for that format to be open source for the community. Then 2d puppeteering artists could use they tool they prefer with a shared file format.
That's presumably because of exactly the problem mentioned in the article. Customers of the other software are forbidden from using those files with other software, which could potentially include the use of a tool to convert that to a better format, so the competing software just made up a different format.