I was wondering, we know that human cells are hard to genetically modify and reinsert in the body?

But what about mitochondria? Being stripped-down bacteria means that they should be easy to manipulate without as much regulation involved or lack of expresion later, so why can't I do a differential centrifugation, transformation, replication and massive reintroduction of mitochondria modified to increase energy production? Or adding a route for antioxidant accumulation? In that way we could increase physical(and possibly neuronal performance) and lifespan at the same time.