I agree. I don't think there's really a case for the US developing any offensive weapons. Geographically, economically and politically, we are not under any sort of credible threat. Maybe AI based missile defense or something, but we already have a completely unjustified arsenal of offensive weapons and a history of using them amorally.
You think the cartels aren't attacking us because we have missiles that can hit Mexico? I don't agree. Somewhat tangentially, the cartels only exist because the US made recreational drugs illegal.
Not sure where the missiles came from, you said all offensive weapons so in my mind I was picturing basic firearms.
Drug trade might be their most profitable business but I think you're missing a whole lot of cultural context by saying the US's policy on drugs is their sole reason for existing. Plenty of cartels deal in sex trafficking, kidnapping, extortion, and even mining and logging today.