Back in those days everyone carried a lunch box, Plus cab was built to use on other vehicles.
But real reason is you need a passenger seat for the lunch box.
Absolutely you do, and here’s a (very expensive) cautionary tale to illustrate why.
Almost brand-new 797 truck. Less than 500 hours IIRC. Operator is travelling loaded to the waste dump and decides he’s feeling a bit peckish but his lunch box is on the floor in front of the passenger seat, not on the seat itself. So he reaches over the centre (transmission shift) console to try to grab a snack from his box, taking his eyes off the road as he did so.
Well unfortunately for him the road was curving to the left, and as a result of failing to follow the curve he crossed over to the right side of the road and made contact with the berm at a speed of somewhere around 25mph. Imagine if you will, a truck loaded to (and maybe well beyond) the Max Rated GVW of 595 tonnes going from 25mph to a dead stop in a distance of about 15 feet.......
The deceleration was so fast that part of the load went clean over the headache rack and landed on the ground in front of the truck. It sheared all the bolts and wiped the LH front leg clean off the chassis and tore off the RH side front wheel by ripping the hub apart through the wheel bearings.
According to the operator “the steering failed”. Yeah, right.