UPX is open source and works on linux , windows and mac (ie. cross platform)
I would like to know why the torrenting space isn't using it already / having a mature discussion about it.
The executable being packed in an executable format means it has to be decompressed on each launch. If it doesn't it means it's not saving any space anyway.
I don't know what packing you're looking for, but Windows applications are typically installed with installers. An executable compressed executable goes against this; unless you want to pack installers.
Traditional file compression works well enough. People know to launch an msi or exe or read a README. Introducing non-standard tools is not necessarily a good idea, and certainly is not intuitive to users not already familiar with it.
Traditional file compression doesn't work that well as compared to upx.
Upx supports as much as 500 mb/s on any reasonable device (this is very comparable to https://github.com/mhx/dwarfs) , which is honestly insane.