They have version control and they can do everything, but it takes effort. Real effort. It's not "here, pick this version and use that". It's "let's use this version as a base and bring out this, this, this, this, <a couple of thousand items>, and that other thing from other versions" plus resolving conflicts, tying things up, and sometimes reworking them because they don't tie up. It's a smaller effort than redoing it from scratch, but it is big enough to be afraid of wasting the time on it - it's on the level of a very big patch (using the entire team worth of dev resources).