Because the more common issues are add-ons, settings and malware and not a corrupted IE.
Did you try IE's Safe Mode?
Did you try IE's reset to default button?
Did you try Grif's ideas with scans with the free apps noted at http://forums.cnet.com/7726-6132_102-5098912.html?tag=posts;msg5099421
Something has corrupted my Internet Explorer. When I try to download another copy and install it, It says it is already installed. How do I re-install the program?