Theres a lot of new texture and model compression technologies that are still in the works and probably haven't been implemented in these games yet. As compression technology gets better, developers are going to have a hard time filling up that space until the end of the PS3's life.
Also in regards to what JB said, the coding might be difficult now, but that will get better over time, the PS2 and its Emotion Engine had similiar coding problems when it first came out but now that is pretty much a null and void issue.