Heyyo,
No what he means is most people don't finish the game cause they usually get frustrated with it, or get a different game. So usually the span of attention to a game isin't too high (well, at least for games in the past this was so..), so developpers know they don't have to make huge storylines and hours upon hours long longevity fun. So in other words:
Average gamer attention span to singleplayer (According to developpers): 5-6Hrs.
Average time to complete the campaign (According to us gamers): 7Hrs at the most.
So you see, they usually try to make games at least longer than our spans, so they think it's a huge campaign n' stuff. Nowadays, most games are getting good storylines and good gameplay, so most people do try and finish them.
Ya know whats sad though? we get short storylines (that's our standard) and for RPG's if the game's less than 20hrs of gameplay it's not worth it. (Many people hate $boxes 'Fable' cause it's beatable in 8hrs...)
If only we could raise our gun-ho standards to a few extra hours....