Let me make it simple for you. Mathematically, AV ends quicker when you do the objectives. HK farmers purposely avoid doing the objectives. They refuse to help defend any of the captured towers, instead choose to constantly regroup, fallback, and turtle. Graveyard farming is not used as a winning strategy in this case, but instead as a way of prolonging the fight to maximize the farm.
If you understand the ebb and flow and momentum of the fights in AV, you will see that when both teams are more or less equally matched, the side that overextends (by having a much greater distance between their graveyard and the frontline) will eventually get overwhelmed forcing them to fall back. That is why unless you happen to have a team that can completely dominate the other, it is good to capture the graveyards. Otherwise you get this constant back and forth (gaining and losing of the same grounds) without accomplishing anything. At best it is inefficient, at worse, if your team is slightly weaker than the other, it becomes a slow bleed to the death with little chance for reversal.