It isn't easy to "just go somewhere else" or "start your own thing" for a variety of reasons. The reality is that the majority of these game developers aren't getting job offers every day and don't have access to the kind of capital necessary to start a company.
It's flippant to suggest that these developers can just find another job, because the other reality is that most of these developers would only be able to find jobs doing the same thing but under a different moniker.
People in other places in this thread have produced some solutions, which I think is somewhere along the lines of organizing and unionizing, and I think that would help for bigger companies who are known to exploit these workers.
Whatever the solution, history has shown that you cannot rely on a company's word that they won't do this anymore.