(More of my wildly popular series on plagiarism.)
Here are some possible causes of plagiarism.
(1) Weak or lax standards in certain fields. There are underlying disciplinary causes of certain lapses. The standard practices in all fields might not be equally rigorous, even if theoretically all scholars are bound by the same code. I won't single any fields out here, but you probably have some idea.
(2) Incompetence. When a student feels he or she simply cannot do the work, plagiarism seems like an option. Buy a paper off the internet! Copy and paste from wiki.
(3) Negligence. When a scholar is poorly trained or does not understand the value of scholarship, or has poor habits of taking notes, he or she may become negligent. By the way, negligence is not an excuse.
(4) Research assistants. Blame it on them! If the scholar is not responsible for every aspect of research, then there needs to be a system for checking the work of subordinates and collaborators.
(5) Misunderstanding of the rules of citation and of their scholarly justification. Some scholars will think that some sorts of plagiarism are ok if they don't violate rules of citation according to narrow, legalistic criteria, or if they plagiarize in the gray areas.
(6) Arrogance. It's not plagiarism if I do it, because I am a full professor.
(7) Means-to-an end thinking. I need this MA to get a higher salary as a teacher; I need to get it done in the most efficient way.
Notice I didn't list other causes. In my experience, people do not plagiarize (usually) because they like someone else's idea so much that they want to take it as their own. People do not plagiarize because they are postmodernists or intertextualists with innovative ideas about intellectual property. It makes me sick when people try to justify it through some recourse to literary theory,* or when others say that obviously the plagiarism wasn't intentional theft of someone's brilliant idea, so it isn't so bad.
The fact that it is easy to copy and paste off the internet cannot justify plagiarism. That just makes it easier: you don't have to type when you can just copy and paste, but it also makes it easier to catch. The internet does not make the prohibition of plagiarism obsolete.
I came of age intellectually during the apogee of Derrida in the American academy, so I know this theory better than you. If you try this argument with me I will crush you.