When I look at the source code of an open source project and see things way more complicated than it needs to be, I just assume the contributors have seen some shit in their lifetime that makes them extra cautious.

This has to be the reason, right? Somebody tell me that there's a reason behind this madness.

I think some just have an idea and stick with it to see if it functions. If it does, then he/she can use their refactoring skills to eliminate the redundancies if they choose to. Another reason could be that the programmer is experiencing this type of problem for the first time, and will utilize the methods described above. Last thing would be over complicating otherwise simple ideas, or algorithms; mainly because of a lack of understanding the framework or the basic principles of the type of language they're using.
P.S *last thing (for real this time) if they weren't aware of a built-in function, they could try to imitate it (solve the same problem (or provide the same functionality) as the built-in) and ultimately make a mess of it.