Why can't you find Ditto in Pokemon GO?

There are a lot of fan theories about this. Some people think that Ditto is hiding out in the form of other Pokemon, or that there is some trick in the game that we haven't found to get a Ditto (or to get Ditto to reveal himself). Ditto is supposedly in the game's code, so it is there, but so far no one has found it. The most likely reason, I think, is that they are going to release an update that incorporates Ditto later, and they just haven't gotten there yet, just like they haven't introduced trading and trainer battles yet.