In the world of speeches and orations, especially historical ones, the persistent misquotation is understandable. You hear a speech. You misremember or mishear a line as something more colorful than it was. If you're a journalist — especially in the pre-recording age, when all notes were taken by hand — you might then type that mis-remembrance into an article. Multiple versions circulate. And so on.
But in the modern age, where basically everything is track-downable, what's our excuse? Why do misquotes arise — and why are they so persistent and hard to eradicate?