Because people do not understand the power of addiction until it has already chewed them up and spit them out.
Addiction grabs you by the balls, takes your body and your mind. You may not even know you're addicted. It's like the black gunk from spiderman three. It gets into your brain like an infection and weaves thoughts into your mind. Excuses, justifications and all sorts of rationalizations, until it has distorted your perception of yourself and reality so far that the only clue you have to anything being wrong other than the symptoms of craving your next fix is the few strange reactions you're able to percieve from other people that let you know you're out of your mind.
In the deepest parts of your spyche when you sit on occasion and think out your situation, you come to the conclusion that you are infact an addict, but that you'd rather be an addict than any other sober individual. You believe that the pleasure of your high is so great that it's worth the pain it cause to you and others, for your happiness in your high outweighs the sum of all misery and joy of the sober world. Ofcourse its absolute horseshyte. But that is the infection that speaks into your mind. If you're lucky, you get an opportunity to realize that your great pleasure is an even greater pain because you experience it in a very short period of time but the rest of the time is a great longing which you cannot percieve as pain as such while you're in it but on observation from a distance is clearly a misery. Therefore it is all total misery and grief, an almost nausiating waste of life and potential happiness, and a truly shameful stupidity, such that I genuinely pity the addict.