1. You are immortal, and therefore, will always be around to help other developers understand and extend your code.
2. You have no life, and therefore, don’t mind spending all of your free time writing code comments and documentation that would already be written in a good framework, so other developers after you (or with you) can also write code for your app(s).
3. You are coding an app that will be one of the most visited sites on the web (all by your self and for eternity if #1 and #2 are true), and therefore, can justify the potential nanoseconds saved by not having the “overhead” of a framework.
4. You are a web security god and are way smarter than the collective group of developers who code and test good frameworks for security issues.
5. You want to spend all of your time re-inventing the wheel by writing your own RESTful routing implementation, ORM, templating engine, etc. etc. instead of what a good framework provides. Or maybe you skip the ORM part and just re-write larges parts of the app when you need to switch DB engines.
6. You don’t do “deadlines”.

If these reasons don’t apply to you, I recommend checking out Laravel, or better yet, the Laravel 4 beta.