The original companies went out of business.. I think 3 different publishers handled the 3 original games. search for "Thief: the Dark Project" if you are curious. (that was the 1st of the 3 original games)
I found a demo on the internet archive.. try this out for a feel of the original if you are curious. - Just remember the download is a 1998 version .. I'm pretty sure the newer graphics and physics are going to be greatly improved.