Imagine if a human needed to make a decision with less than a few seconds of reaction time, at freeway speeds, and the LTE network where the truck happened to be at that moment was having a hiccup.
It's much worse than a cargo drone that might have a 12-14 satellite GPS+GLONASS lock, that could be programmed to stop and hover in place, or retrace its flight path breadcrumb style back to base in the event of brief loss of WAN network connectivity. It's something with possibly 20,000 kilograms of mass moving at 70 mph with live humans in traffic all around it, and a lot of inertia.
It's an external technical problem that the company itself can't solve, because they're not in control of LTE/5G network operators, they don't make LTE radios/modems/antennas, etc. The only places that I'm aware of using fully remote driven trucks are giant open pit mines in western Australia, where the mining company fully owns and operates in-house the network linking the trucks.
I personally know very little about self driving technology, but working in network engineering, the idea of relying upon the normal publicly-accessible nationwide LTE network operators for something that could be a life-critical safety function really scares me. Even with a max-priority service like Firstnet on ATT it would be sketchy.