Not everyone feels compelled to "beat the game"...some people actually just like playing the game. There doesn't need to be an endgame. It's not a themepark game, there is no rush to cap and then "endgame"...you play and exist in sandbox games, not become the best in them and beat them. If you get bored, you do something else that maybe seems interesting. But if everything bores you, then stop playing...the game is no longer fun, it happens. It's like the pvp focused players who just can't understand crafting people.."why would you want to just sit and pick grass and craft things all day in game????" ..people actually like that. Some people just like logging into games like this, sitting around a campfire and chatting with friends or guildmates and doing nothing at all *GASP!*.... no endgame sir.
*edit*: That aside, remember, the developers vision is to release new areas, new crafts, new things to do, etc as time goes on. Some people will get bored faster than others, it happens in all games. Show me one game that has managed to keep all of it's players for years and years by releasing "endgame".