Can developers add things from mods to their servers (vanilla)?

[maxon-69 35]

Why don't developers take ready-made items from (free) mods?

Can they contact the mod developers and ask for permission? 

The essence of the question is to combine the efforts of people who make cool mods with the main game. After all, developers can free themselves from routine work and focus on other things.

If this happened , we would enjoy a great variety of content. And oh, yes - we would have had a helicopter. What prevents this? The concept of private servers?

[nemorus 266]

They can but they can't. It's all about their official status. Professionals taking content from amateurs? What would neighbors think? It would prove two things at least: that dev team is interested in co-op with their costumers and also that dev team can't figure out stuff on their own. Besides, using mods as a way to make money is not a good idea. Strategy First is a "good" example of what may happen if mods are used like that. Vanilla is lacking big time feature-wise but dev team must figure it out on their own for their own sake. They've gained enough resources to do just that.