Press Esc to exit full screen mode.

We considered it, and chose to leave the behavior as-is.  It was carefully designed to address a number of security concerns, and while it sounds like a simple thing to change, it's not when considered in the context of the universe of possible attacks.

In essence, the full-screen prompt exists to prevent a malicious actor from drawing a pixel-perfect simulated browser, for the purpose of phishing.  If you can draw on the whole screen, you can draw a convincing browser lock icon (which is what tells you that you're talking to the server that you think you're talking to, like your bank).

This important mitigation is not going away.  The web is a hostile place, and security necessarily outweighs convenience in that context.   I know that we've similarly considered a "remember" checkbox, but also found that infeasible because of how things work in practice.

If you're developing content and want a fluid full-screen approach, a desktop application build with Adobe AIR would allow you to migrate that content to a desktop application, where this kind of phishing attempt is a non-issue.