Because they are ignorant, end of the story.
The myth that Madonna does 'nothing', 'she doesn't write her music', 'she's just a marketing product', 'she's a whore', 'she can't sing' etc has spread through the years, and people don't feel the need to dig deeper into Madonna's ACTUAL story & catalogue; it's sad but true, I was talking with someone I know about this not long ago..