In nature, diversity is a sign of health and strength. We strive to keep biological habitats diverse for this reason. When diversity is lacking the ecosystem suffers as disease causes more damage. Since humans are part of the natural world, diversity is also health and strength for communities.
Many of your linked "papers" are a common mistake called causation doesn't imply causation. You specifically linked to studies researching countries where ethnic diversity correlated to negative factors without leaving a reason why ethnic diversity might be the direct cause. You didn't deliver.
The lack of a common unified culture means people can only relate to people based on universal values. People often mistakenly project their cultural beliefs onto other people as universal values, taking them for granted. It leaves only our most basic instincts to relate to people with.