I also tend not to play other games until I've fully cleared the one I'm currently on.
Another reason is that I simply don't have the energy to juggle multiple games like I used to
I stick to just one game at a time. I learned this the hard way: I once tried playing more than one and became obsessed with a specific title (usually the newest one), leaving the other aside for days or even weeks after I'd actually finished it. When I finally went back to it, the magic was gone—I'd lost the desire to play, the thread of the story, and even my place in the game. I'd end up finishing it reluctantly and out of boredom, just so I wouldn't leave it incomplete; that's why I prefer to focus on just one.
Although... I have tried it a few times. If one game is very short and the other is dragging on, it doesn't take me long to take a break from the longer one and return to it with renewed enthusiasm.
I play multiple and it takes forever for me to finish a game. I like the freedom of being able to start and stop whenever, and I save very often. I also mainly play mobage so I don't have a choice there