_You_ may not see a problem when Some Company uses your code in ways that are not covered by the license you and I are operating under.
However, _I_ surely do. The Free Software movement is based on equality. I get your code and can modify and enhance it, you get to do the same thing with mine, and thereby we all benefit. That's the key reason why the GPL exists.
You only need copyright assignment for one single reason, and that's to create and sell something that contains my code but is not bound by the license. I, as a lowly contributor, may not do that. Only the assignee can. That's a fundamental inequality, and it cannot be fixed by forking.