Because we depend on biodiversity to survive. It's what allows animals to not be wiped out by only one super parasite. It's what allow plants to bread so that they get robust and nutritious. It's what allow all our bodies to not be vulnerable to only one disease and all die.
It also supports the balance of all the ecosystems: forests, seas, rivers... Which we also depend on to survive.
It's what makes us live on a beautiful, comfortable planet instead of survive in a terraformed rock we have to fight against while rushing to the next scientific way to palliate yet a new deficiency.
In our planet system, everything is connected.
Let's take a very naive but simple to understand example, the tomatoe.
Planting so much, and always the same kind of tomatoes makes the land nutritionally poor and the plant susceptible to parasites. We compensate this unnatural state by addition artificial nutrition and pesticides to the plant.
The result ?
Most tomatoes are tasteless garbage that looks more like a plastic ball than a fruit. Their nutritional content has been divided by 10 during the last 80 years, so that you need to eat much more to remotely get the same minerals and vitamins. When you eat them, you also absorb a small amount of all the products we put on them, with consequences we still don't know about.
That's just for a simple tomato. Bring together all the food, the water, the air, the temperature, etc. together, and that's how much we are disturbing.
It's not just emotional. We are seated on branch we are sawing.