Why is virtue the only good for Stoics?

The core argument for Stoicism

Lotus flower against a dark background (unsplash)

If Stoicism was known for one position in the ancient world, it was the seemingly outrageous claim that virtue (inner strength: courage, moderation, justice, wisdom) is the only good. Surely, Aristotle is more reasonable. Alongside the whole world, he adds to virtue some external goods to make life easier, and enable a person to do more.