Because politics is complicated, and that discourages most people from bothering to understand it. It's much easier just to go along with what other people around you are saying.
I hope you research your opinions before you express them at some point in your life.
It doesn't help when he doesn't follow politics at all
I thought the plagiarism thing was pretty desperate. "Yes We Can" was used by Deval Patrick in his campaign for governor of Massachusetts (he won). Patrick has apparently worked with Obama and gave him permission to use the slogan, and either way, it's not like "Yes We Can" is an academic paper or a book or anything where it would actually matter if he plagiarized it.