Ask HN: Why aren't more companies source code leaked?

Why is the source code of Google or Facebook not leaked more often? With so many engineers it only take on laptop or one rouge developer to leak it somewhere anonymously.

I know of course there would be serious repercussions for the person doing it, but I just wonder why accident haven't happend more often? Like what is the change with 10000 engineers that not one engineer would be unlucky and have their laptop stolen.

I know that Facebook source code was leaked but that was back in 2007.