Why don't open source licenses give a more specific definition of "derivative works" to reduce ambiguity?

Question

The Free Software Foundation is of the opinion that a program that links a library is a derivative work of that library, so a program that links to a GPL-licensed library would need to be distributed under the terms of the GPL. That is what I understood from the following posts:

• What are the arguments for considering dynamic links to constitute derivative works?
• Is it allowed to dynamically link a GPL licensed library for commercial use?

However, the Eclipse Software Foundation has the opposite interpretation. In the Eclipse Public License 2.0 FAQ, merely using the interface of a library does not create a derivative work:

5.3. I’m programmer not a lawyer, can you give me a clear-cut example of when something is or is not a derivative work?

If you have made a copy of existing Eclipse code and made a few minor revisions to it, that is a derivative work. If you’ve written your own Eclipse Platform plug-in with 100% your own code to implement functionality not currently in Eclipse, then it is not a derivative work. Scenarios between those two extremes will require you to seek the advice of your own legal counsel in deciding whether your program constitutes a derivative work.

For clarity, merely interfacing or interoperating with Eclipse Platform plug-in APIs (without modification) does not make an Eclipse Platform plug-in a derivative work.

My question is: why are interpretations of "derivative works" confined to FAQs instead of being placed directly into the license text? If the definition of what constitutes "derivative works" were to be explicitly stated in the license text, there would be less need for all these opinions and interpretations. Why not explicitly state what "derivative works" includes and excludes like in the Apache License 2.0?

"Derivative Works" shall mean any work, whether in Source or Object form, that is based on (or derived from) the Work and for which the editorial revisions, annotations, elaborations, or other modifications represent, as a whole, an original work of authorship. For the purposes of this License, Derivative Works shall not include works that remain separable from, or merely link (or bind by name) to the interfaces of, the Work and Derivative Works thereof.

Answer 1

Because open source licenses are copyright licenses, and copyright license do not get to define their scope as they choose; or more precisely there is a "maximum size boundary" to which the license can apply, with any attempts to reach beyond that boundary are null and void because the copyright in one work does not apply beyond that boundary.

While the precise size of that boundary and the specific terminology used varies across jurisdictions, there is a general concept that copyright applies only to works which are in a general sense "based on" the original work - for example, I can't claim that because I once wrote a program using the variable name i that all programs using the variable name i are subject to my copyright; that's too big a reach in any jurisdiction.

Of course, what licenses can do if they wish is to reduce the size of that boundary, and exempt things which are legally derivative works from the provisions of the license. The Apache license as you note above does explicitly reduce its scope as you note above, while the GPL chooses not to do this. Both of these decisions are in line with the goals of the sponsoring organisations (the ASF and the FSF).