Do you have a specific reason in mind? I have no experience in this area, so my own speculations are just that. One practical issue may be lag. Connectivity would also of course be very important.
There are historical reasons as well, but given we're discussing new technology (automated trucking), there are plenty of things that will change as a result and may not have much sway going forward.
I'm not familiar with the actual design decisions involved, but I'd also imagine field of view for an on-the-spot operator would be better than for someone operating through cameras, unless you invest in a very expensive surround setup.
There are historical reasons as well, but given we're discussing new technology (automated trucking), there are plenty of things that will change as a result and may not have much sway going forward.