They had it and took it out. The way the AI evaluates trades, it wouldn't matter anyway. No matter how logical such a trade might be to a human, the AI would never accept it.
the AI's never really accepted these in GC2 or GC3 either. However, humans did use them to give away planets that were causing problems (especially ones in an AI's ZOC). With colonies these days, it makes more sense than ever. Makes sense that this option should go back in.
Starbases were also not tradable for a while - I haven't looked lately to see if they now are. But those too..
edit: you beat me to it!
A pity, as sometimes it seems you never know where a planet of yours will end up, and trading a planet that ends up in another civ's territory seems like a good idea.
Those were my thoughts exactly..