I haven't ever finished a GTA game. I always get half to three quarters of the way then get bored. And only turn it on to get into huge chases then turn it off.
Same happened with Red Dead Redepmtion. Rockstar cannot keep me interested lol.
It's kinda the same for me, although I did finish VC and I think SA. But then again, there haven't been many games I've been interested in for a long time. Somewhere after two months I usually stop playing the same game for a while.