Companies don't want to give us good games anymore.
Good doesnīt mean lengthy, and a lengthy game can be awful if poorly designed. That being said, i would love to have a good game that lasted for 15-20 hours. I blame the online component. RE4 was bigger than 5 because it didnīt have co op or any other form of online play. Nowadays developers just assume that by slapping some co op or competitive modes on a game, they can have 8 hour campaigns. But 12 hours is still pretty good by default, if we take a look at CoD, whose main campaign lasts for 3 hours (and many other games suffer from this).
But yeah, they should make this game 5-6 hours per character, averaging the game duration between 15-18 hours, plus some extra mini story ā la Separate Ways, mercs and possibly raid mode. Donīt forget there are also unlockables, new costumes and weapons to buy AND customize, which would boost the gameīs length.