Here is the reason: There are about 300 possible form of ID that could be used. TSA looks at
1. The person matches the photo.
2. The ID matches the name on the boarding pass.
3. The boarding pass is dated right
4. The ID is not expired
5. IT is a valid ID.

Numbers 4 and 5 above take the most time as the location of date of expiration is different from one to the other. There are some other aspects that I am not allowed to discuss.
NDSOL has it right though. TSA is limited to how many TSO's they can have at any airport. Now TSA has to check ID and in order to do that it might come at the cost of a whole screening lane being closed in order to put TSO's at the ticket checking posts. On a lighter side though the program is suspended for now.