They'll get down voted and lose their rep, so they'll get no recognition and no reward which are two of the main factors of plagiarism in the first place.

There are also bots like @cheetah that detects similar content, and you can report stolen content and other abuse to @steemcleaners.

Genuine people have a monetary incentive to keep Steemit clean as allowing bad actors to thrive on the platform decrease the value of the platform which can influence the value of the rewards negatively.