Because any exponential growth rate will eventually reach the physical limitations (i.e. carrying capacity) of the underlying processes that produce goods, whether that is fish or molybdenum. Eventually, even if we can keep scaling exponential growth by spreading to other planets and stars, the growth is fundamentally limited by the speed of light, which means expansion in space is limited by O(n^3), and oh yes, even O(1.00000tiny^n) will eventually grow faster than O(n^3).
Yes, I've watched Albert Bartlett's lecture several times; and if we get to talking about converting literally everything to pure computronium in a sphere expanding in all directions at approximately light speed, I admit that's a lot further into "forever" than I was imagining.
I was more thinking of the near-term future of resource scarcity, to make the point that "value" doesn't run out just because gold or oil extraction from the Earth runs out; people value intangible things and the economy runs on that being real value.
https://bollocks2012.wordpress.com/2013/04/22/the-greatest-s...