A better question to ask is why the hell to the drugs in Fallout 3 and New Vegas have little to no downside. hardcoe mode give some downside.
Yeah, that is a good question. IMO, that's even worse than using real-world drugs.
"Hey kids, in Fallout, you can pump whatever crap you want into your system with no negative after-effects! JUST LIKE IN REAL LIFE! BECOME AN ADDICT TODAY!"
Edit: my bad, meant to put this in my last post.