I didn't say it was harder to play defense with a lead.
But It's not necessarily easier to play defense with a small and early lead. A small lead early in the game doesn't change what the opponents offense will do, and doesn't somehow prevent the opponent's offense from scoring.
I think the problem here is you are only looking at extremes - an all out shootout, or a low scoring, grind it out game on both sides. And, really, even if that were the case, neck and neck to the end is neck and neck to the end, regardless of whether both teams have 30+ points or 20- points.
But the reality is there is a whole lot of room in between those extremes, and having a good defense doesn't somehow mean you have to have an offense that isn't capable of anything more than to "grind out a game". Seattle, for example, won largely on their defense, but they still had Russell Wilson passing the ball.
Ultimately, being able to achieve the best possible combination of scoring and preventing the opponent from scoring is what works. That can mean either the offense or the defense is a little stronger, or it can mean a good balance of the two.