The person who originates the thread is already listed right under the thread title when you're viewing a list of threads. It would make no sense to do this under the "Last post" column.

Also, most people actually read the whole thread instead of just replying to the latest post. It seems like one wouldn't have very much context if one only looked at the last post in a thread without going back to its beginning to see what the actual thread was about.

DocJohn