Have you ever written software before? It's really challenging to allow others into your code base especially when you have a lot of users (I consider this game to have a lot of users). The new person could completely butcher the project, sabotage it, or just introduce bugs. Even if they work out perfectly, it could take them a really long time to get up to speed. So the game dev would have to spend countless hours just ramping them up in order to get them to a point where they're productive. You have to really trust the person too. And in most cases, they'll expect to get paid. So it's not just as simple as letting others into the code.
I do a lot of coding, yeah. What i am talking about is moves, items, etc. Things like that can be coded as separate classes or functions or whatever. Whoever was going to help would not need to have full access to the game code lol