I doubt it's an act of carelessness, anyone with that kind of a balance would at least know how and where to input the appropriate fees.
You assume that all mistakes come from the lack of knowledge or experience. But there's a wide variety of other reasons that can force someone to make a mistake, like being tired, sleep-deprived, distracted, etc. I don't see anything strange about a supposedly experienced a wealthy user making such mistake.
If anything, it shows how dangerous Bitcoin can be, because it allows such things to happen. So this should be a reminder that with Bitcoin you are your own bank, meaning you are responsible for checking your transactions and no one else.
Especially since we see those mistakes happening with fiat all the time, which should be simpler to use as you do not have to input the fees you have to pay, as the fees are subtracted automatically by the bank, and while there could be something going on it seems to me the most likely explanation is that the person behind the transaction made a major mistake and they simply did not realized this was the case until it was too late, and now they depend on the kindness of strangers to recover their money.