For games where the code _is_ the difficult part (Dwarf Fortress, etc), its probably so complicated that having the code helps nothing, unless you want an exact copy (at which point, just pirate the game).
The number of applications or games where having access to the code helps even somewhat to do anything is vanishingly small.
@Lmaydev @db0 the source is typically the least important part of any game. Games with any amount of success get copied overnight by game farms; no need for code access.
Even more: if I need to copy a game, observing it is enough, I don't need to deal with the certainly messy original code that I don't understand well. Rewriting from scratch will certainly be faster than deciphering a 3rd party codebase.
The hard part is almost never the code, it's design, gameplay, graphics, theming...