Fantasy football is a marvelous game, but it isn't an accurate representation of the on-field product.
Defenses are usually lumped up into one "DEF" unit, and we assign points for non-scoring stats such as yards and receptions. This allows us to enjoy the fantasy game more and celebrate when a player rips off a big play -- even if it doesn't end with a touchdown dance.
This, in part, also allows us to rely on quarterbacks who weren't even on our teams the week before because of how many starting quarterbacks are available late in drafts and on your waiver wire all season long.