A ton of these things is handwritten assembly. "All" you need to do is run it through a disassembler and you got the source... the key problem more is that virtually all cracks and keygens come heavily packed.

Either to protect the authors of the original software to check if the warez group got the full algorithm or if they have something mildly different from the original that allows the authors to detect a keygen in newer versions, or because the warez group wants to make life more difficult for copycats... or, and I've seen my fair share of that (and earned good money to clean up), a third party wrapped a highly popular keygen like for Adobe CS6 in some sort of malware and wanted to avoid detection.

Nothing a debugger can't fix. UPX and PKlite packing made keygens and cracks look more sus than they were. It source had been available, that would make them appear even less sus. There were a few drama incidents with Razor where a cracker provided source because of accusations.