well because:
1. they dont know any problems they will run into
2. they might get another "emergency" job while working on your cars
3. parts might be outta stock
the list goes on

i see this same thing at my job, i work in a pharmacy and tell people it will be about 15-20 mins to get your rx ready. they come back and its not ready and get mad at me because i quoted the time too short. i cant determine when we get phone calls or insurance problems