Because mental funds are non-fungible. Amortising health costs through progressive income tax helps a) keep overall costs down, as the state centrally organises the health system / collectively negotiates with providers and b) is more fair. It doesn't feel like you're being utterly screwed through no fault of your own - e.g.: medical bill after an accident or violent attack. Instead society at large acknowledges two things a) we all need medical care and b) our need is largely disconnected from our capacity to pay.