Because since we were young in elementary they have brainwashed us to believe that. Now people are living thinking that Weed is one of the worst drugs out there, when in reality it is not.
I used to be one of those sheeps that hated anything to do with weed and I disliked being around people that did it. Now that I am informed and did my research I wish I would have never been that way. Weed is awesome. I do believe that any one under the age of 18 should not be allowed to smoke it.
Another thing is that society believes that weed makes you lazy. Nope you were already lazy. I don't ever want to be considered to be a stereotypical pot head... The ones that have no job and stay at home being bums, or the dumb little annoying immature kids acting stupid on the streets while high.
I know many people that are very successful and they smoke weed. Many of them use it as a "reward" after a long and stressful day at work or what ever. I'm pretty much that way too. I work, pay my bills, own my own car etc etc... but I do smoke, and I'm happy doing so
Also like many have said, if you feel like it's "unhealthy" to smoke it then there is millions of other ways to use weed. Vape, edibles, oils etc etc.....