Why is it easier to write than to speak?

I ask to myself why writing is easier than speaking. I'm learning English and for the last three months I have been talking to native people. It's a very good exercise because every day I can understand more and more words. However, when I have to (<em>not necessarily bad, just not necessary</em>) talk I have to think carefully of all the things I want to say and sometimes I'm not able to remember the word that I need. It's frustrating! When I listen to a native speaker I think, "Yes, that's exactly what I want to say," but when I have to do it... I hope that some day I can speak English if not in a perfect way, then in an almost in a acceptable way :(

I am sure that with more time and practice you'll be speaking English MORE than acceptably :)