I'd like to add that the jump from 2GB to 8GB is a 400% increase in RAM. It's not a trivial difference.
8GB is just the amount of RAM that has been empirically determined (nobody starts counting how much memory each component of the system will use before writing the OS - except maybe on some embedded systems) not to cause problems for typical uses.
To extend jkh's analogy, what you're asking is "Why does the Earth need 23 hours 56 minutes my-orbital-mechanics-professor-would-kill-me-for-not-knowing seconds to turn on its axis and not 12 hours?"
The answer is: "Well, if you could analyze every interaction the Earth has had since its creation, you'd have your answer."
The difference is that in this case, it is possible (though laborious) to account for everything - but you can't expect a detailed analysis to be made a priori, especially when the many components come from tons of different sources who'd each have to publish detailed memory usage results.