The reason why we don't see monocultures in nature is because they put every single one of their members at risk of sudden death. A single pathogen can wipe them out. The exhaustion of resources required to sustain them can starve them. The build up of by products can kill them. Think of a medieval town drowning in excrement, running out of top soil and about to get hit by the plague.
An ecology on the other hand puts none of their members at risk by virtue of their existence because they are diluted. Think of a band of hunter gatherers, they might individually die at 40, be exposed to ebola and worse, but the fact they are so widely separated and so few means that they have few if any vulnerabilities that can kill the whole tribe.
The reason why we see monocultures now is because they are more efficient and our time horizons are tiny. Amazon will fail catastrophically. Billions or trillions will be lost. No one will learn anything because it's cheaper not to than to build a really resilient network.