Depends on how you define trivial. It'll take a different input circuit or a (if not even more complex to make it automatic instead of...) manual input voltage switch that depends on user intervention and at least a couple additional parts, where parts cost money. Manufacturers usually source the PSU with a very cost conscious attitude and it almost becomes an oversight so long as it works and doesn't violate any regional safety standards.
If you wanted to purchase an aftermarket PSU that's full input range, has the right voltage and current capacity of course, then all that remains is having the right output plug on it, assuming they didn't pull a stunt where there's an IC communication with the host that makes it behave abnormally if a genuine OEM PSU isn't detected.
Dell laptops come to mind as an example of that, but I haven't looked into whether anyone else implements such a thing. It wouldn't surprise me at all Lexmark printers also do it, given the past experiences I've had with them using similar (actually worse) chip protection even for the effin' LCD display boards, just to keep customers from using perfectly good new/pulled parts if not bought through their channels at full retail pricing. [/rant]
So no, there isn't a good reason, but there is a cost reason, plus they have to bundle each with the respective wall socket plug, plug terminated cable, or additional modular cable.