By design, the NFL is meant to be a league of parity. The league rewards bad teams with, at least in theory, an easier schedule than the division winners, access to higher draft picks, and more.
So, in a theoretical world, the difference between the haves and the have-nots shouldn’t be that large, and the collective talent levels shouldn’t vary too much from team to team. As a result, each year there are many games decided by a single score (eight points or less). The good teams find ways to win those close games, while the lesser teams find ways to lose those contests.