I think it's because so many sites rely on very simplistic log-in systems for the sake of simplicity. Sometimes it's easier to just make a basic thing on your own than it is to use somebody else's system.
I'd imagine a few open-source php classes and one or two standard DB tables would do the trick, and maybe a simple interface for managing users and emails.
I'm not much of a coder but this seems like it might be easy enough to try out. I might give this a go, if it's not done by the summer (which is when I have free time again).