It is true the patterns could be excerpts from the GPL work. But they need not be functional or even intelligible as anything other than as part of instructions, e.g., short strings of octal values representing characters, with wildcard characters interspersed.
Patching might even operate by only using line and column numbers. The patch itself in that case need not contain any text from the GPL work. It might be just an index into some file, a list of line numbers and columns and any new text to be inserted.
The point is that the process of patching need not necessarily reveal anything about the original GPL work. The editing could theroretically be done in such a way to make this separation.
Anyway, we have drifted from the original question which was , essentially, why do programmers not distribute instructions on how to modify GPL source code or additional source code that can be linked with GPL source separately from the GPL source code.
The reason I asked is because what I see is that original GPL source code is sometimes "hidden" in commercial products by virtue of being bundled with some "derivative work". To me this obscures the value of the original GPL source code in favor of the closed source derivative work, which may or may not be as valuable. NB The usage of the words value and valuable is purely subjective. Opinions may differ.
I am not sure I understand the comment about BSD licensing. Assume for example that the goal of the license is to require attribution, how does one avoid that requirement without changing the license? The licensor can opt not to enforce the license and may accept payment. But the license still requires what it says.