You distribute books all over the country? Amazon can do it better and cheaper.
You print books? Well we have e-books now.
You have a massive back catalog? Google just scanned all of it.
You do marketing? Authors now have their own followings on social media and can reach them directly.
You give cash advances? Fans on Kickstarter give 10x more.
What's sad though is that publishers have historically had one power that would have been unassailable – editorial judgement. They could have sustained their brands on quality. Imagine a world where you wanted something good to read, and among all the garbage out there saw a title and went "this one is by Simon & Schuster, it has to be good". Instead they went all in on pulp bestsellers and celebrity memoirs at the expense of actual good authors, and here we are.