I would guess it's due to investment...

Self driving experts can easily pull in salaries of $5M plus per year, and this stuff takes many years. To build a team of those people, you need to burn an insane amount of cash.

That much cash is only available from investors if the reward is huge. Replacing all interstate trucking is a much smaller potential reward than replacing all human transportation.

Hence everyone is working on the harder, but much higher reward projects.