The source is rock solid, I know the peeps behind it. They have actually testified before Congress on their findings. They certainly don't have every power outage attributable to squirrels counted, but the 3 attributable to humans is accurate.
As for my quite a few reasons......
1. In all of recorded history no one has beaten the squirrels without a real gun.
2. The squirrels have 24/7. They don't have to go to work for example.
3. They are far more motivated than you. This is a game to you, it's life or death for them.
4. They are smarter than you.
At some point, soon, you are going to lose power at your house. You have been warned.