I can see your point. From a fan's point of view, we just want someone to remake X-Com (or MoM or whatever) and we don't care how they do it and why they do it and just get more annoyed by more unfinished projects.
While you could just probably have one project for that, in most cases, each project has their own take or approach or things they'd like to see in a remake, and each project goes a different way even if they're all trying to remake X-Com at heart. A lot of people aren't encouraged unless they make the project their own special thing which they have input over, and wouldn't really work getting together with other projects with different goals. Forcing different teams to merge might just make the end result even less productive.
What the players end up choosing is another matter. Different strokes for different people and all that, otherwise we'd only have a single program for every single task.
In any case, while OpenXcom and X-Com Reloaded don't have that problem since they are pretty similar, they're still very early in development and might go different ways eventually. Plus everyone codes their own way, so code isn't really "mergable". You can try to learn from it and apply that knowledge in your own project, but at this point, for both projects to merge one of them would pretty much have to throw their code away and work on the other. A noble goal maybe, but there's no way I can force anyone to do that, we all love our projects dearly. Maybe if we had met up earlier things would be different. I hope you understand.