Thats because buildings have not been designed for a small chunks to simply break away and fall down. Its done by one giant piece at a time with particle left behind. This could be improved in another year of game development but there is so much more work. That takes every chunk in the game, divides it by 8, and fixes textures. Too much work.
Look at this post also:
http://forum.1cpublishing.eu/showthread.php?t=15244