- this is cool, but I could build something better (how many 37signal open source clones are there)
- this is cool, but way too expensive (Github complaints)
- this is cool, but let me use Google AdWords to get free upgrades (DropBox)
- this writing is great, but I'm blocking all the ads on the page (daringfireball)
Another problem is that when developers decide to do their own startup, the only domain they really understand is software development.
There are millions of people who have problems who can't code - building another bug tracker, productivity tool, email management app, GTD widget might be fun, but the economy of real "business" software that's out there is far larger and more lucrative.