Because since 1983, she has made some of the greatest songs, albums, videos, concerts, etc. in the history of pop music. Just listen to The Immaculate Collection for an example. Then, pop in GHV2. Then, Something to Remember. Any pop star would be ecstatic to have one YEAR of her career let alone 21.
And, if she was just great because of the marketing and the imaging, people wouldn't buy so many of her albums and singles. Lots of people have great image, but no talent to back it up and sustain a twentieth of Madonna's career.
What makes her music great besidess simply fantastic pop writing is that she uses humor and she provokes thought. The same is true for her concerts and videos. She is not flawless and fully admits it. But, she is the real deal. She has an energy that is electrifying. When she walks into a room--other celebrities stop and look. She is dynamite.
Her acting career is where the flaws abound, but she has done some good work on the stage and screen, and nobody can say she isn't fascinating, even at her worst.