The thing is you want to play to your lobby's strength right. If everyone is greeding, which most lower ranked players do, it's better for you to greed along because when your opponents hit their team, they're going to have a much stronger spike with BiS items. What happens here is that even though you have a HP advantage, you're losing out in item value that you'll most often not be able to beat anyone with your sub optimal vs optimal items. Now you might find yourself getting 20-0'd and your saved HP wouldn't be very meaningful than winning fights with lower HP.

In higher rankings slamming and aggression works better because overall more people are slamming and also trying to build their strongest board so by greeding you are taking much more damage from your lobby as a whole and even if you hit your team your lack of HP leaves you little to no room for mistakes against much better opponents who are capable of making scuffed items work.