I had the same thing happen, never found out why either. I just manually uninstalled the game.
Directions to manually uninstall.
This is only my second post....but it is probably the most important one I've ever made............I can't thank you enough for your step by step instructions on how to manually uninstall the TS3 game. I'm not a modder, nor do I like to mess with files I'm not familiar with...however in this case I made an exception. Thanks to your valuable instructions, I was able to uninstall and then reinstall the game. This time, however I've made a back up of all the files.....not just selected ones. Again....I am much indebted to you....thank you!!