The clever little cold virus keeps mutating and taking on different shapes so that anibodies created from the last cold are ineffective to the next one.
I heard somewhere that there are about eighty common varieties of the virus, so if we get one a year we will be immune when we get to eighty-one.
It is not in drug company's or chemists' financial interest to invent a cure. Have a look at the number of cold remadies next time you go to the phamasist.