But let's look at this constructive post by LoyceV, for example:
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That took him seconds to write.
I can beat that: a simple "no" can be enough as an answer, as long as it comes from knowledgeable user.
Constructive does not equal a lot of text, a short sentence that hits the nail on the head is also constructive.
There's the problem, you can't hit that nail if you don't know what you're talking about.
For the record: I've also made posts that took many hours during multiple days.
Alternative, non psychoanalytical hypothesis: it's easier.
In the spirit of short answers, I'd answer the question "Why people plagiarize" with one word: Money!