Why shouldn't I quote famous scientists in my SOP?
A statement of purpose is forward-looking. It is not meant to be, to paraphrase Wordsworth, "recollections of early childhood." I don't really care why you decided to study mathematics when you were seven years old, nor do I care about some generic quote from a scientist that inspired you. I want to know what you might want to study as a PhD student, and why you are motivated to study that specific project.

If a famous scientist said something relevant about your proposed project, that's a different story, because it's actually significant to what you want to do in the future. Otherwise, leave it out—it just annoys most of the referees who will eventually read it.