Natural resources certainly exist, and the destruction of natural resources has an obvious real-world effect.

So why do economists apparently prefer to deal with fictions than realities? I suppose one reason is that once upon a time capital would have been based on some notional and desirable physical incarnation of value such as gold. Or cowrie shells. Or wives. But today the notion that capital has a physical basis is obsolete. It's purely a social tradition. You can still buy things with capital, but that's only because everyone agrees to play the game and not ask questions about how it's played.

That's why you won't find a rational underpinning to economics and to ideas like GDP. It's like trying to find a rational reason for the pyramids.

To me the constant quest for 'growth' looks like any other irrational ritualistic anthroplogical activity. Humans seem to get stuck into these collective belief systems based on simple primate desires for more status and more resources - like pyramid building or 'growth' - and continue until the local ecosystem falls apart or some other drastic change happens. Otherwise there's no one outside of the system to point out that in most ways the beliefs and actions are really rather silly, if not actively destructive and pointless in a monkey-trap stupid kind of a way.

It's true there's a rather vague and inconsistent conflation of 'growth' with personal freedom that's often used a justification it. But we're on much shakier ground there, because it's hard to prove that this personal freedom is universal - for most of the world it hasn't been - or that it's not a kind of semi-voluntary ritualistic slavery, even among those it's supposedly benefiting.