I’m not knowledgeable about odometers at all, but if it’s as easy as “hooking up to a computer” to get the real mileage, why don’t people do that when they buy the trucks?
For example, on a VAG car, most generic diagnostics scanners just report the mileage as it's recorded in non-volatile memory in ECU/odometer/transmission (which people know how to change).
But a specialized scanner (VAG COM, for example) also displays freeze-frame information of diagnostic faults (and this sometimes includes mileage, depending on the module).
If the people doing corrections change the mileage reported by major modules that contain this information but do not bother to erase diagnostic faults, you can sometimes find the real mileage, as recorded by electric seat module which detected undervoltage condition when the battery was removed, or headlight module when the bulb burned out.
Agree, but not everyone here can afford <5 year old vehicle, so people have to do with the means they have. As soon as warranty/lease ends, people generally go with cheaper, non-manufacturer maintenance shops.