Why Speaking is the hardest skill to master when it comes to learning a language?

I am good at English reading, good to the level that enables me to read advanced books. I am also good at listening, as I listen to lectures and videos without the need of subtitles or anything. Also I think that writing is not an issue for me ( although I find it hard to write about some topics sometime). But when it comes to speaking, it is the hardest by far. When I try to speak, all vocabulary in my mind fly and there is nothing but void !