One of the most fun theories I've heard came from a friend of mine. He believes Satoshi hasn't been found because there isn't a Satoshi, but rather that it was

One of the most fun theories I've heard came from a friend of mine. He believes Satoshi hasn't been found because there isn't a Satoshi, but rather that it was a code name for a major research project. Creating, designing and building the first version of Bitcoin is complicated and spans a number of fields of expertise. Most of these fields of expertise only have a few truly exceptional practitioners who would be capable of the original thought. You then have to factor in the peculiar nature of: 1) Not touching his new wealth of initial Bitcoins. 2) Disappearing into obscurity. 3) Being able to avoid being discovered (or nobody actually trying that hard to find him). Occam's Razor principle would suggest all of that is less likely to have some altruistic single brilliant individual create Bitcoin and then leave. It's more likely a team of individuals would create it under a pseudonym, dissolve after completion of the project and then no longer have access to the accounts/keys. Large government agencies would have the ability to bring top minds together, and less interest in liquidating the initial Satoshi Bitcoins. Why would they want it? There are plenty of reasons governments would want a public ledger system. Unlike some of Bitcoin's spin-offs, Bitcoin isn't actually really private at all. It would be great for tracking and stopping crime. Of course this is all a theory that sounds like it's out of a spy novel, but I think it's good fun. The other likely possibilities is that Satoshi could have been older and passed away, or he truly is some brilliant mind who is well hidden.