The clipboard is a program. Why can the applications that want the data not make an API or file available that the clipboard uses to pass the data into, and the

The clipboard is a program. Why can the applications that want the data not make an API or file available that the clipboard uses to pass the data into, and then the app does what it will with the data? e.g. echo "this is on the clipboard" | cat - cat doesn't grab anything, it receives via a standard API, both its own and the pipe. Now imagine echo is a clipboard program that exposes no API that cat can use to access data at cat's behest. Note: As this was informative I hereby declare this a non-useless use of cat.