One reason is that there are new medications that treat the flu and make it go away sooner. These treatments do not work on a cold. And these treatments must be started within the first forty-eight hours of coming down with the flu or they don't work. You can see from the chart that the flu usually hits fast and hits hard with lots of headache, muscle and joint pain and high fever.
Colds are more head congestion, scratch throat, and dry cough that come on over a day or so.
Fever may be hard to detect because we all take so many medicines with Tylenol, aspirin, Motrin, Advil, etc. It might be a good idea to check your temperature before taking any sinus or pain medicine to see if you are running a fever so you can decide better what you have and what to do. Your doctor may prescribe a pill or a medicine you inhale into your lungs if you have the flu. These medications have been shown to shorten the time you suffer with the flu by a day or so. They are somewhat expensive and you will have to decide if it is worth it to you or not.