I'm not a trucker, but I used to play one between choo-choo gigs. I drove a tractor-trailer, maybe a total of 18 months, over a 20-year span.
There are PLENTY of sound, reliable trucks out there...or were, when I was fooling around. You could see them working non-common-carrier jobs...say, jockeying construction trailers around, or pulling carneys' trailers, loaded with knocked-down Ferris Wheels...that kind of work. Hauling a farm's produce to a local resorting spot.
What puts those trucks off the road, is INSURANCE. Getting insurance on something older than a certain cutoff...used to be, ten years, it varies with various edicts and stat spreadsheets the bean-counters get...but you just cannot get insurance.
Right now, you've got two kinds of OTR operators: Fleets, and O/O truckers. The owner/operators buy their insurance, either in the market or they get coverage through whoever they're contracted to. The carriers who contract owner-operators don't want to see older trucks out there...in their shriveled mind, old translates to unreliable and service delays. Individual performance doesn't count. Plus, of course...this Safety Cult mindset has trickled in. Airbags for TRUCKS, FFS? I couldn't believe it, but it's true. I look at the cab interiors of these new trucks, padded car-steering-wheels, automatic transmissions...I can't believe it. I learned how to drive on a 1975 Kenworth conventional...it was only a generation removed from the "Duel" truck used in that classic movie. Looked very similar, except it was clean and in good order. Painted metal dash, Bakelite huge steering wheel, NO power steering (as late as 1990, NO big rigs had power steering). Stewart-Warner white-needle-on-black-background gauges.
Those trucks get junked for insurance reasons. Plus, now, CARB won't certify/allow-operation of, diesel engines that were fine by CARB just a few years ago. The incredible cost of replacing a huge diesel for POLITICAL (junk-science) reasons, in an old frame (the diesel engine is the most expensive component in that truck)...the numbers don't add up. Banks and finance companies are not going to loan to buy a new engine, but they'll eagerly finance a new truck.