Well, source code is not sth that you "crack", you can only reverse engineer it (I think it was done with Doom, also OpenRA) or steal it from the company's servers. The use for it is also rather niche, so the risk vs gains ratio is not attractive enough to feed dedicated websites. You can also look at fully open source games like 0AD and check out what they did!
Edit: I stand corrected (thrice); Doom was indeed open-sourced, not reverse engineered. Thanks for pointing out!