If ground and neutral are bonded at main panel, why aren't all grounded device chassis energized?
In a perfect world with conductors that have zero resistance, neutral would be always at zero volts everywhere, with respect to ground. It has current flowing through it, but that's not the same as having a nonzero voltage, which I take to be what you mean by "energized".

In this world, that's not totally true, but the point where neutral and ground are connected together is ALSO the point where there is a physical grounding rod driven into the earth. So even if some part(s) of neutral are significantly above 0V, due to high resistance, or a fault, or whatever -- the point where neutral and ground are connected should still be very close to 0V, which means that the rest of the ground wire should still be very close to 0V, as long as you don't connect it to neutral anywhere else.

If you connect neutral to ground somewhere else, away from the grounding rod, this no longer holds -- in the event that neutral were to break, current could flow through the ground wire near the additional connection between ground and neutral.

There are different systems used in other countries and situations, and the best way to set up a protective grounding system depends on what assumptions you make about failure modes. In a perfect world where the neutral conductor has zero resistance and never breaks, you wouldn't need a separate protective ground wire at all. So ultimately you need a model of what kinds of failures you expect, in order to decide what kind of protection will be most useful.