Because generally drugs that make you feel better than normal have enough negative physical effects to be "not worth it".
That may generally be true, but it's certainly true of nicotine, alcohol, and caffiene; and it's probably not true about marijuana.
And plenty of people feel that way about the drugs perscribed to make them "better"; I took both Celexa and Wellbutrin for depression. Celexa made me feel bland; Wellbutrin wrung my brain out like a dirty sponge. And all for what? It wasn't like I was thinking about killing myself; I was just tired, listless, didn't feel happy about my life; etc.
I understand that you're making a point about the stated goals of pharmacology; in pratice those goals are not really met.