It's not so much signing away your rights as getting something in exchange for something. As you say, you can always fork the project, but very often large projects (that tend to be the ones with CLAs) are also used by companies that rely on them. What you get in exchange for the CLA is your code (written to scratch your itch) becoming part of the mainline, and saving you the trouble of merging with every update. If scratching your own itch is the goal, and if your contribution is small relative to the total project (and it often is for projects with CLAs), then granting the project leaders a joint ownership over copyright is a very small price to pay in exchange for having the leaders take your contribution into the mainline. If, on the other hand, your goal is to shape the project, this usually entails a large number of significant contributions, and if you do that, becoming a part of the leadership or getting a part of the project ownership can be negotiated.