This complicates the state of a development folder. If Dropbox went down mid upload or you hibernated your computer before it had completely uploaded (it happens), you then work on another computer and that uploads some files, suddenly you are going to get a merge conflict which Dropbox cannot easily help you with.
Software development is not one file being edited, its a collection of files in a particular state that give it meaning. Dropbox is good, its gotten better but its still not the right tool for the job.
Git (or any DVCS) is good enough that you can commit and push every 5mins if you wanted. The point is that the state a a whole makes sense.
Plus what about when you start working with other developers, you cannot all work in Dropbox, you'll get merge conflicts every day. What about bug finding features like bisect? Or just having a log of when thing happened?