What do kills give you? Gold & exp.
What does minion farm give you? Gold & exp.
What gives you an edge over your opponent? Higher level & more items. How do you get those? Gold and experience.
What do objectives give you? Gold, stat buffs, map control.
So tell me, what's less risky to do in most situations? Chasing kills or farming minions?
Are kills more valuable than stat buffs and map control?
Theres a time and place for trades, poking your neck out to kill every person you see is stupid. For example, if you're trying to grab baron or a drake, its valuable for you to kill the enemy jungler or at the very least harass him & make him use his summoners so he cant try to steal it. On the other hand, if you dont do anything after killing him then its almost a complete waste of an opportunity.