Because their shapes are similar, you mean? Like, could you substitute bamboo for sugar cane, because they look similar? Or - and this has happened (but certainly not to us) - could you substitute salt for sugar?

Orzo is a pasta, shaped like little tiny footballs or fat grains of rice. Arborio rice is a plumper rice, but rice nonetheless. If you are making a risotto (one of the highest uses for Arborio rice), you need the starch that oozes out of the grains of rice as they absorb the cooking liquid. Orzo would not ooze, and depending on the amount of cooking liquid you used, you would either wind up with well-cooked orzo or orzo soup - but not anything resembling a risotto.

The name orzo means barley in Italian, and is a reference to its general shape and size, but you can't substitute orzo for barley, either.