"Publishers" doesn't really mean an awful lot. Valve is a publisher. GOG.com is a publisher. If you mean Ye Olde Video Game Publishers like EA, then...why?
I mean traditional video game publishers that are part of the entire process, not just the actual publishing of the finished game. Otherwise almost every indie developer would also be a publisher. Valve and GOG only cover bringing finished games to the market, while a real publisher provides funding for development, builds developer studios, provides tools and manpower, covers marketing, and so on.