well, they could, but in most cases the capx posted have issues in them or are incomplete (I personally recommand to post a description of the issue, and the capx with altered graphics, or another capx with the same issue so there is no stealing risks), however to make profit out of it, they would need a C2 license and working on your codebase to complete the game, which takes sometimes more time than doing the game themselves.
however be wary if your project is something you do not want to share the entire source.