It is bad because Allah forbade it. That is enough.

Alcohol, for example, has a lot of negative affects. It can be addictive, it can cause various health problems, being drunk can lead to poor decision-making, and so on. However, none of those reasons are why we refrain from alcohol. We refrain from alcohol for Allah's sake. It does not matter if there is some benefit to alcohol -- indeed, the Qur'an tells us that there is some good, but the bad outweighs the good. But measuring the relative "goodness" or "badness" of drinking alcohol is generally a stupid exercise. In my opinion, it doesn't matter what the worldly "good" is, or if drinking alcohol can be justified through worldly arguments; what exactly is "good" or "bad" about it isn't the point. Allah has forbidden it, and that is enough.