A browser isn't just a bunch of code that runs on your local machine. Security, maintainability, etc. depend on a development organization behind it, and these needs will only increase as the usage scales. You really need an ecosystem, not just an app.

And speaking of ecosystems, will your browser use plug-ins? Will it be compatible with other plug-in standards? If not, that's a whole different hill to climb, convincing the wider community to develop for your platform.

In other words, it takes a whole lot more than coding to launch an app, especially an app as complex and as central a part of a user's life as a browser.