In the forum Some guy Make a gravity for solids...I download it But It dont work SO if someone wil make som funkcion mod here is download link http://www.box.net/shared/yuu5tfdvk8
Please read the forum rules on the Announcements board
The games engine is based on per particle movement and checks each particle to get information from it and act accordingly. Moveable solids would require either a full rewrite of the base system OR it would be an incredibly buggy implementation which would involve a lot of checks and a significant slow-down in the game.
That's why. Now read the rules, use the search and check the wiki before posting further suggestions.