Some of the 10 ton straight trucks have auto transmissions.
The only ones I’ve driven are Chevys.
When the truck is driven off road, construction sites, farm lanes etc the auto transmission does not “know what to do” when you cross a soft spot.
You’d be surprised how quickly the engine speed varies.
I’ve yet to see an automatic with more than one reverse speed,that doesn’t mean there aren’t any though,which is handy in backing into unpaved work sites.The older Macks have 5 reverse speeds.
No one has mentioned shifting rear ends yet. The big trucks sometimes shift rear end speeds three times in 13 speeds if you include granny.