Because while raiding works, in the long run commerce generally proves to be more profitable. It requires less labor and involves less risk, and it's open to a wider range of entrepreneurs because it doesn't emphasize personal physical capabilities. And it can (and will) be done in stages. Even barter societies usually come to recognize this eventually.
And that's why we find, for example, copper sourced from the Great Lakes in artifacts from ancient cultures in what's now Mexico. Salable goods were traded along the river systems of the Midwest until they reached the Mississippi (where they could be concentrated in Cahokia, for the period when that was a thing), and gradually made their way south. It would have been absurd for, say, the Aztec to try to raid Michigan - completely impractical. But trade along that route is entirely feasible.