One reason for post-scarcity to not exist is that the more resources we have, the more we will reproduce which will end up in consuming more and more resources. It degrades the ecosystem and brings down the quality of living for everyone involved.

This is pretty much what is happening around us. As we get hold of better resources, we reproduce more to make it impossible to have everything for everyone.

Again, we will have to re-discover the definition of scarcity because we have too many people unhappy with the system.