tax, title, and license, inspection and certification for a long haul truck is much more involved than an automobile. For example, a 2019 Volvo VNL might only cost 165,000usd. However the total cost in the first year for owning this truck (assuming the person you hired to drive it doesnt wreck it) is more than a Rolls Royce Phantom...close to half a million. The engine also wont realize the actual MPG advertised until its broken in...roughly 10-25k miles.

You wont realize the gas savings in the first 10 years, or ever really, so most trucking companies rarely care about new trucks with emissions controls that dont save fuel. They get rebates from the federal government for installing those metal spoilers on trailers anyhow.

The operating costs of modern emissions equipment is far greater than the cost of the extra fuel wasted on a less efficient drive-train. It might be close to even for operators that do maintenance in house (I'm not familiar enough with the industry to know). If emissions equipment wasn't expensive and unreliable (relative to the rest of the truck) this market wouldn't exist.

Capex vs Opex? The cost of new/refurbished equipment is an up-front cost and takes longer to depreciate (but refurbished is lower-cost and so has fewer immediate penalties), whereas diesel fuel and other similar costs can be charged immediately as expenses related to customer fees. Same as why businesses might prefer to pay more real-money over the long term to rent servers via AWS than spend money up front for servers and run their own, longer-term-cheaper, server farm.

(Edit: clarification)

Might be less efficient, but some older diesel engines have a track record of being much more reliable.