You want the frame to flex. Stiff boxed frames will just crack or tear out a weld. Semi's have a ton of flex in the chassis.
Can you find any examples of cracked boxed frame? (except for corroded Toyota frames)
Number one rule for chassis engineering is to design flex, damping, and compliance into the SUSPENSION, not chassis/frame. It's next to impossible to accurately predict the dynamic behavior of a flexible frame - its effective spring rate and natural frequency is always changing depending on load and what's bolted onto it.
Do you think semis do it by choice? Or simply because it's impossible to resist 36,000 lb-ft of moment (2000 lb-ft engine * 18:1 first gear ratio)? Why don't tour coaches flex? They too use "semi" engines and their axle loads are even higher (54,000 lbs on 3 axles). Perhaps their monocoque structure is way stiff in torsional rigidity?
Fully boxed frames can be hydorformed. And use less steel to get the same strength. So they're cheaper to produce, that's the only real reason they exist. The rest is just hype and marketing.
Cheaper only in material cost. R&D, manufacturing (tooling) is much, much higher than a C-channel.
Also corrosion is a concern. (Toyota) I can service both sides of a C channel frame. Can't really reach inside a hydroformed frame. If I had it my way, the new Superduty would be C channel from nose to tail, with C channel cross members.
That is a valid point, hence why chassis cabs will always be made out of C-channels, for ease of bolting to the flanges.
Corrosion is always a concern, but the same applies to unibodies. But we have made large strides in anti-corrosion technology in the past 20 years.
A lot of people look at semi tractors and say: they use C-channels, and look how strong they are. But few actually look at the metallurgy in detail. Class 8s don't just slap on any random c-channel, they use what I consider extreme-tensile strength steel.
And old pickup (80s/90s) with c-channel used steel around 20,000 - 30,000 psi in strength (rebar strength). Modern HD pickups use around 35,000 psi, until the 2013 Ram HD, the first to hit 50,000 psi. This is the start of "high tensile strength". Medium duty trucks use around 70,000 psi steel, or "ultra high tensile".
A Volvo tractor? Try 120,000 psi!
So while a c-channel frame can be very strong, it's cost and weight prohibitive to do so at the pickup scale. There's a reason everyone (except Toyota) have moved to fully boxed frames on pickup trucks, and it's not "marketing", it's the most reasonable method to achieve high strength, rigidity, payload, and NVH due to less flex and low frequency resonance.