why doesn't this occur as a configuration?

I think those guys you mentioned typically max them out around a 44ton capacity in which can be done with a heavy spec 3 axle truck. XL Specialized builds a 50ton capacity model in addition has a neck long enough for a 4 axle with lighter steer. In the end it will net more payload that away. The biggest thing I see is CA people do not like using 4 axle trucks and where you see 4 axle trucks with trunnion compliant laws would be a state like TX. But TX people don't like mechanical necks which is 99.9% of the trunnions. Just have to put them together to come up with the combination that you are talking about.

But I have actually seen a 4 axle truck with a stationary 10' wide trunnion Cozad before (non-expando). It was owned by Tri-Dal in Southlake, TX. They have (or had) 2 of them.

 

No he’s talking about recognizing trunnion axle setups.

 

The word Trunnion goes to Naval Guns back in days of sail, you have the barrel of the weapon and on that barrel is two very heavy duty pegs to each side resting in fit notches on the naval carriages or land caisson. That is the trunnion. Helps the barrel rise and fall according to aiming screw or wedge under the breech. As well as keeping control of the barrel through the firing recoil. Transfers the impact to the carriage.