True, but if you research lots of sources and still emit significant blocks of verbatim text without attribution, it’s still plagiarism. At least that’s how human authors are judged.
This is a reductionist take. Maybe it's not illegal per se where you live, but it always have ramifications, and these ramifications affect your future a whole lot.
Scale might be a factor, but it's not the only one. Your neighbor might not care if you steal a grass stalk in its lawn, and feel powerless if you're the bloody dictator of the country which wastes tremendous amount of resources in socially useless whims thanks to overwhelming taxes.
But most people don't want to live in permanent mental distress due to shame of past action or fear of rebellion, I guess.
It likely comes from the saying similar to this one: "kill a few, you are a murderer. Kill millions, you are a conqueror".
More generally, we tend to view number of causalities in war as a large number, and not as the sum of every tragedies that it represent and that we perceive when fewer people die.
