Back in the days it was done by good design and gripping story. you wouldn't want to put the game down and would play until dawn to experience every little aspect of it. I don't even remember how many times I finished Fallout 2 in totally different ways or stayed up late to get that one perfect kill in Hitman: Codename 47. No achievements involved.

Now you are artificially attached to the game collecting all those hidden packages, intels, pidgeons, orbs, skulls or whatever the developer put in to keep you going.

If the game world is interesting enough you don't need the carrot on a rope to go off the beaten path and explore the environment. A recent example here is S.T.A.L.K.E.R. It has some areas in the game that never are part of any mission but still are lovingly put together and well detailed. It makes the experience your own by challenging your imagination in the same way a good book or even a painting makes you wonder "what else is in there" other than the obvious main subject. 

But the developers are not the only ones to blame. We, the audience have so many games on so many platforms to choose from aside from our other hobbies, that we have slowly turned ourselves into a picky mob with ADD. Our attention span is so low, that we constantly need to be padded on the head and intensified by the current activity. Be it something that adds to your gamerscore, level in an RPG or any other form of a presentable score (=E-peen).

I myself am a part of it too. Often grinding through same tedious tasks in a game to get those 25p.

When did we start working in our games?