Quotes use space that you do not have
Personal statements are short. Every sentence needs to help universities understand your motivation, preparation and suitability for the course.
A quotation does none of those things. It tells the reader what somebody else said, not what you think, what you have learnt or why you are applying.
You could spend a sentence introducing a quote, another sentence giving the quote itself, and then a third sentence explaining it. That is a large amount of space to spend on an idea that would be stronger if written in your own words from the start.
Admissions tutors want your thoughts
Universities are assessing you, not the author, scientist, politician or public figure you have chosen to quote.
If a book, article, lecture or idea influenced you, write about the influence directly. Explain what interested you, what challenged your assumptions, what question it raised, or how it changed your understanding of the subject.
For example, if a particular economic argument changed the way you think about inequality, discuss the argument and your reaction to it. The quotation itself adds nothing. The interesting part is your response.
Famous quotes do not make a statement memorable
Students often choose quotations because they sound intelligent, inspiring or impressive.
Admissions tutors have read thousands of personal statements. A quotation from Albert Einstein, Jane Austen, Stephen Hawking or Nelson Mandela does not make an application stand out. It signals that a student found a quotation and decided to include it.
The statements that stand out are the ones that contain specific evidence of subject engagement and thoughtful reflection. A paragraph explaining how a research project changed your view of a topic is far more memorable than a famous quotation about learning, discovery or success.
A quote is never your strongest sentence
Students sometimes ask whether there is an exception for a particularly powerful, famous or relevant quotation.
There is not.
Whatever point the quote is making, you are better off making that point yourself and then explaining why it matters to your interest in the subject.
Your personal statement should show how you think. A quotation shows how someone else thinks.
Including a quote does not strengthen a personal statement. Leaving it out does.