Because the incentives are not aligned. In a capitalist society societal consciousness is necessarily imprinted with capitalist (value extracting) logic.
If most of the people around you believe that making money is the logical way to survive then making money will become the goal of many to most human interactions. People will choose big publishers because big publishers guarantee them some amount of money (and services around publishing) in exchange for the ownership and monetisation rights (and then the right to sue somebody into oblivion when they break copyright law).
So the rich and vibrant ecosystem can be reality, just not within current society.
If most of the people around you believe that making money is the logical way to survive then making money will become the goal of many to most human interactions. People will choose big publishers because big publishers guarantee them some amount of money (and services around publishing) in exchange for the ownership and monetisation rights (and then the right to sue somebody into oblivion when they break copyright law).
So the rich and vibrant ecosystem can be reality, just not within current society.