The essay is meant to be a reflection of *you* and your unique experiences, voice, and perspective. A professionally written essay, even if it's excellent, won't showcase those things. Admissions committees are looking for authenticity. They want to see your writing style, your thought process, and your ability to tell your own story. A ghostwritten essay defeats the entire purpose.
While you can seek help with editing and proofreading from teachers, counselors, or tutors, the actual writing and ideas should be your own.