It's not that we don't know what causes it (it's a rhinovirus), it's just that virii are really hard to deal with as they live and replicate in your cells whereas bacteria float around on their own and are generally easier to knock out because of this. Treating a virus is like having to burn your house down to get rid of the roaches because they're in the walls...this is kinda what a fever does, virii are generally kind of fragile, so if you jack up the temp in the "house" they don't replicate as well. AIDS drugs don't treat the virus itself, they block certain enzymes necessary for the virus to replicate. This is one of the reasons they have so many side effects as your body needs these enzymes for day-to-day stuff as well.
The other big problem with colds is that the virus is constantly mutating and changing so even if you identified a strain and made a vaccination for it, you'd just get whacked by another strain you weren't immunized against. That's why there's a new flu vaccine combo every year....new flu bugs every year.
Fausto