The answer to this question officially has everything to do with the idea that laws are meant not only to protect citizens' rights but to ensure citizens' safety as well.
Some of the trickier aspects of how we draw the line between what is safe and what is not are pointed out in the posts above. Clearly, there is plenty of room for debate as to where the line should be drawn but very few people really think that there should be no line at all and that all controlled substances should be legal.
Some drugs are not illegal. I am assuming you are talking about illegal drugs only. Illegal drugs are illegal because they are capable of causing a lot of harm to people (so are many prescription drugs). Prescription drugs are used for people with some sort of medical condition. Illegal drugs are not. Illegal drugs are for recreational use and they have many devastating or even deadly side effects.
Inferring that you are speaking strictly of contraband narcotics, here's the "why:" Illegal drugs have been proven to be detrimental to human health in epidemic proportions. Meth, for instance, is addictive on the first use, and ruins the lives of its users. Marijuana, as a general rule, has a more cumulative negativity -- it takes longer for habitual users to be "ruled" by their addiction, but once they are, their brains and lives are decimated by its effects.
Similar things could be said of other "street drugs:" crack, angel dust, and the whole host of prevalent illegal narcotics have one thing in common -- they all result in personal destruction.
This is not a well-phrased question. "Drugs" are not illegal. Some drugs, like caffeine, are completely legal; some drugs, like nicotine, alcohol, and pharmaceuticals are controlled -- you have to meet certain conditions in order to obtain them, like turning 18 or having a prescription; and some drugs, like marijuana, LSD, and crystal methamphetamine are banned.
Among the banned drugs, some, like crystal meth, create a danger to the person with no medical benefit. Some create a danger to others, like PCP. Some controlled drugs, like salvia in California or alcohol, have no medical benefit and are at least as dangerous as some banned drugs, but these types of drugs are heavily supported by money from the industries that produce them, and will never be banned.
We assume that everyone knows what is best and will never act willingly in ways that will harm him or her. Unfortunately this assumption is far from true.
We all recognize the need to guide children and even insist on their taking or not taking certain types of action. This is because we believe that children do not always know what is good for them. In similar way adults need to be protected from harming themselves.
Drugs is one thing that poses maximum danger of this type. It can lure people of all ages to get into kind of problems which are fairly well known. Drugs, in addition to harming the health of the people who take it, also causes other problems like poverty and crimes. Also, drugs taken by some people also attracts other people towards it and in this way can become a widespread problem in society.
This is the reason for making drugs illegal and taking strict action for controlling its sale and use.
Illegal drugs or narcotics can be addictive. The word "addictive" means that a person will want to keep taking the drug, even if it is making them sick. They can also be dangerous because they change the way in which other drugs work. For this reason, many governments have laws which regulate their use. Very often, they are illegal, if used outside a medical context.
What drugs are considered legal, and which ones are considered illegal varies from country to country. The punishments also vary.
I believe, like alcohol, some drugs such as marijuana will eventually become legal. Once the government finds a way to get taxes and the people make enough noise, it will cross over.