Because that's not exactly how it works. Nature is finite not because it doesn't try to grow limitlessly, it's that nature grows in equilibrium with the resources and space available in the system/environment.

That often results in the same thing, but the distinction is very important, as if tomorrow there suddenly were infinite resources and space, nature wouldn't just sit there and go "nah, I don't do infinite growth", it would quickly adapt to just grow endlessly.

On a smaller scale, this is what happens with algae blooms before they run out of resources and die off back into equilibrium with their environment.

The only reason complex creatures can even exist is because of genes made to restrain this behaviour in an organism's own cells, turn those off and you get the cellular equivalent of an algae bloom in your body.

Even then, plenty of organisms do that very same thing, like the rabbit swarms in Australia or Locust swarms in various parts of the world.

Nature doesn't aim for infinity, but it's not not aiming for it either. If the environment allowed, that's what would happen.