Players don't finish games because they get bored, or they can't. Not because they don't have time. Because something else is more interesting.
Red Dead Redemption is an excellent example! I love GTA-style games. I fully expected to love RDR. I played about 10 hours and then felt I had done everything that I cared to and quit.
It was boring. The open world was too open, and not enough world. Travel took too long and had nothing interesting along the way. Quests were simple and boring, and didn't require any thought.
It might have gotten better, but I'll never know because I got bored and found something better.
Other games (GTA San An, Oblivion, FO3, FO:NV) held me to the end and then some. They kept me interested the whole time and I kept coming back for more, no matter how busy my schedule got.