First, in a serious campaign, major speeches are checked and re-checked and re-re-checked. I don't give a talk to 10 colleagues without going over it 4 or 5 times.
Second, Ms. Trump said she wrote it herself which is clearly not true and which the campaign manager denied.
Third, people hear things, remember them, then years later the words come back and sometimes they don't realize it's something they heard before. Happens. What you do in that case is apologize, admit error, can the person who SHOULD have checked. Do that, the story goes away, a low level staffer goofed. What you do NOT do is double down, insist it's not plagiarized, and blame someone else.
Fourth, the embarrassment is that for 8 years we have heard both Obamas demonized, Michelle was "angry black woman", some RWNJ sites even say she's a man in disguise, kid you not. So this horrible despicable un-American person is the one whose words you crib? Either she's not really horrible or you just copied from a horrible despicable un-American person. When the whole world is watching, a serious person does not make those kinds of flubs.