there are two reasons, one has already been noted elsewhere - that you don't know what will succeed so you cast your net wide and hope you got one of the few winners in your catch.

The second reason is that your diverse catalog exists also as advertising for that subset of readers who will become your next authors, they might be dreaming for years about the day they can be published by Random House, just like their hero, obscure writer X from some years ago who did not earn out, but ended up inspiring the next generation's big thing.