They do if you are automating them to avoid having to pay human drivers. It's kind of silly to go to all of the effort to automate a truck and then make someone sit on their thumbs behind the wheel for hours on end.
If you can reduce legal risk(accidents) by 50x or 100x and extend the road time by 2x or 3x, then paying a security person seems like a doable call. Especially if you can pass some of that premium risk mitigation on to the customer, if load value dictates.
Raw hourly cost may not even be the primary point under the manpower line of reasoning. It is certainly important, but not necessarily the key issue.
You can't increase road time with a human driver at the wheel, that's a safety issue.
While it's already dubious to hire a driver for an autonomous truck, it's even sillier to pretend that the driver is useful when you're having the truck do the work while the driver sleeps. Extending road time by 3x means running the truck 24 hours a day, and keeping a person awake that long will not improve safety.
Raw hourly cost may not even be the primary point under the manpower line of reasoning. It is certainly important, but not necessarily the key issue.