Because they got money, plain an simple.
Illustration art is a better and more sane business than actual modern art is, though it's more competitive. Illustration usually does go by skill to make realistic or stylistic drawings for things like book covers, posters, comic books, character commissions, ect.
In modern art it's more sporadic. Things can be considered good art whether the artist has skill or not, it all matters on what catches the patrons eye or what they think have meaning. Sometimes it doesn't even have to do with the art, it has to do with the artist. An example of that would be Andy Warhol; he got popular more off his personality than he did off his art.
Either way, both modern art and illustration runs off money and what the people with money want. Even though illustration is more skill based it's not as recognized as fine art is because illustration relates to media and entertainment.