To begin with, in a nutshell, filmmaking is broken down into three parts. Pre-production, Production, and Post-production. Distribution is the last part, way down on the timeline and doesn’t come into play until everything is in the can. However, if you are ever going to get to distribution you will need to spend a great deal of time in the Pre-production phase.
There are times when you don’t have that luxury, especially if you are shooting on the fly, but more often than not a film can take years to make. This can keep it in the Pre-production process much longer.