Why offer an $800 iPad that can do the work of a $1000 Mac? One word: profit.
It's been reported that Apple's services division is, all by itself, the equivalent of a Fortune 100 company. The overwhelming majority of that service revenue is coming from the iOS side of the empire, not the Mac. A $2000 MacBook Pro might do a better job of filling the corporate coffers at the point of initial purchase, but there's a far smaller pool of potential customers to make that purchase. An $800 iPad Pro has both a larger customer pool and a greater potential for service revenue over the course of its lifecycle. By changing the purchase model in this way Apple protects their enviable profit margins while lowering the cost of entry, which is essential for continued global growth.