I think that's a very good question, actually.
There is no doubt that all writers do derive a lot of ideas from the works of others as well as from their own lives & experiences.
Fortunately, I think most writers are not particularly prone to suing or being sued unless the copying is so absolutely blatant as to be indisputable.
After all, any court case is going to take up a lot of time and money with what amounts to the word of one writer against that of another. There are so many similarities in fiction anyway that providing absolute proof is all but impossible.
So I guess the short answer would be - very few authors are sued because the chance of winning is so slim.
Also, most authors don't make huge amounts of money so it isn't really worth it.
This, at least, would be my guess <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />