And if I were to reverse-engineer a binary then I could still add that code to my software.
That's actually an important factor for ancient software whose source code was lost. A developer could, for example, declare all their old Atari 2600 games to be under GPL by just announcing it in their news blog. Collectors could then hunt for the binary files and decompile them. Decompiled software is still a derivative work, so that source code would still be under GPL. Sadly I'm just aware of one case from years ago where I can't even remember the specifics who and which software it was but he was like "I found some floppy disks from the 1980s, I lost the source code but binaries under GPL, so have fun".