You have those that grow out of the hobby. Often finding a better outlet or a different game that suits them better. I know my long time DM Brian really was always a WarHammer 40K player, he just didn't know it. I suspect the same from one of my other players. My friend Derek and I could get our writing or acting fix elsewhere. Then there those going with the flow that are not so set on gaming as they are spending time with friends.
This sort of gets me where I was headed. The gaming friendship is sort of an odd one. You are often playing with people you may or may not consider friends. This is because the hobby doesn't appeal to everyone and if your best buddies are life long gamers you are lucky. Often those of us that are more mercenary will leave groups of friend for other gaming groups that offer something more in terms of the roleplaying experience even if the players are people we actively dislike. I really can't think of situations were I put up with bigger weirdos and freaks than in my roleplaying life, well perhaps my family.
Here again you are dealing with a less than committed relationship you are perhaps more committed to the game world and the story than the relationships with the people at the table. This often leads to player unconcerned with jumping games. You seldom see the years long game in this setting.
Something I suggest is going beyond the game and building friendships with these people. Take a night or two out of the game and do something else. Bowl! Take a few minutes out of each game and talk about what you each want from the next game. Be honest and ask the hard questions. Are you having fun, what would make you have fun since you are not. What worked in this last session what didn't. In essence you should be the bartender at the end of the night with an ear for your players. Say little as you get more out of it when you listen. Let that form your next game session. Every one's experience will be better for it.