Others have pointed some reasons, but there's also another important one. Copyleft is a promise to he end-users that - no matter the circumstances - they won't

Others have pointed some reasons, but there's also another important one. Copyleft is a promise to he end-users that - no matter the circumstances - they won't be stuck with whatever they were served, but would be able to maintain or hire someone to maintain. It's not only freedom to adapt and modify the code, but also - when a project is a library - a freedom to not be stuck with ancient bug-ridden insecure builds if there are newer versions. So if you ask "why copyleft?" think how much abandonware with statically linked libraries (e.g. ancient OpenSSL) you've seen.