Why do projects fail?

Project managers will appreciate this:  http://youtu.be/OEQWSjWSx4Q Warning: you may laugh. (Also, you may cry.)

Partial transcript:

Boss:  I need to work on a sexier project to boost my career
Boss:  One that will sound good on my resume and not fail until i get a better job
Boss:  Hey -- how about nanotechnology stem cells for fighting terrorists?
Dilbert:  Uh, ok...

Dilbert: This is the bare minimum budget i need for my project.
Boss: Hm. What could you do with half that?
Dilbert: Fail.
Boss: When can you start?
Dilbert: Think I just did.

Dilbert: I need to contact vendors for ballpark prices to see if the idea is feasible.
Boss: You can't talk to vendors until the full board approves the project
Dilbert: But that would require a cost-benefit analysis, and i can't do that without ballpark prices from vendors...