> the pressurised can analogy kind of works if you scale it up?

It did 4 billion years ago when the debris in our Sun's accretion disk coalesced to form our planet, and again when whatever planet sized object hit us to form our Moon, but since then we've been cooling off like a pot of old coffee. Fortunately there's a lot of mass left to cool off, and it's stored in the best Thermos ever created...