Except they probably couldn't, because the carriers are going to lease self-driving trucks from specialized agencies that will exist exactly for that purpose, as they won't want the administrative costs of dealing with individual owners (which makes some sense when it's an owner/operator as an alternative to hiring a driver and buying/leasing a truck separately, but none when the truck is self-operating.)

So, since no one is going to be likely to pay them for use of the self-driving truck, there's no anticipated income stream with which to finance the purchase.