Because a lot of the symptoms of common viral and bacterial illnesses are not caused by the infection, they're caused by our immune system ramping up to fight the infection. Many of them even serve a constructive purpose, or are side effects of stuff that serves a constructive purpose. Examples:

• Fever - turning up the thermostat can inhibit reproduction in some pathogens
• Congestion - increased mucus production protects mucus membranes in the respiratory tract and transports debris (dead cells, mostly) away from the respiratory tract faster. Sore throat is mostly a side effect of mucus draining down the back of your throat (into your esophagus, and the extra mucus in the stomach can cause upset stomach, or at least diminished appetite).
• Fatigue and body aches - A Defcon-1 immune system is energy-intensive. You're tired because you're spending that energy, but also because the fatigue signals conscious you to stop doing stuff and go rest so your body can spend its energy fighting the infection.