/u/alvar368 , I should note that even Medicare has an annual deductible. It's a small amount ($226), but it still exists.

I would agree that moral hazard is the real reason. Without a deductible, there's no downside preventing patients from acting in bad faith and getting care when it isn't really necessary, such as if they just have a common cold.

Health insurance is still doing its job even if it only helps in catastrophic situations where you would owe tens of thousands of dollars because it prevents you from going bankrupt with an Out Of Pocket Maximum. A few thousand dollars is still a lot of money, but it generally isn't an amount that will lead to bankruptcy.