I've seen a lot more of this lately also unfortunately. I suspect it has to do with experience and elo. People become self absorbed or something. I'm stuck with this also right now.
One thing I've learned to do when I carry a game is once my build is good enough is to give the kills to my teammates since cash becomes less important to a well fed carry, and much more important to cash strapped teammates. Having 3/4 of the kills on 1 champ is questionably effective.
So I usually end games with tons of Assists, and die less since chasing rarely helps.