This is an EA game. The development team has to consider the entirety of the game and balance. Modders don't have to worry about that, they just take someone elses work and add their own. It's easy to add content from a mod, without considering what is coming down the line and balancing.

If they were to take these modders on to their team, it would likely result in a worse product that will take longer to complete because they have to consider the rest of the game and work on different parts.

I use to script for games in C++ back in the day, and it's just far to easy to add/edit your own mod than to actually work on the game and balance.