The problem is thinking anything cures the cold or flu. Once you have either, you have it until it runs its course. The only way to cure either would be to completely eliminate them or how they function in the body with medicine that doesn't currently exist.
This is a common misconception of the placebo effect. The placebo effect is a measurement issue, not an actual benefit.
Tests are corrupted by using the reposnes and judgement of humans. People will say they had some sort of benefit because of expectations, poor recollection and politeness. It doesn’t mean a benefit was gained. A placebo group allows researchers to quantify how much the placebo effect has on the data they gathered, they can then see if the experiment they did had any effect. Placebo is literally our definition of zero effect.
Anyone telling you placebo is a good thing is wrong, misinformed or deliberately misleading you. In many countries it is illegal for doctors to prescribe ‘placebo treatments’. They will still recommend such things to their patients - not because they work but because they get the patient out the door and less likely to come bother them again.