- Can you buy new fire insurance to cover house that has already burned down?
- Can you buy new life insurance to cover somebody after they already died?
- Can you buy new auto insurance to cover a car that has already been in an accident?
It doesn't make sense to force insurance companies to cover pre-existing medical conditions because if people know they would be covered for pre-existing medical conditions then there is no reason to buy insurance before they get a medical condition.
Of course if you forced everybody to buy health insurance then you could cover pre-existing medical conditions. However, forcing somebody to buy something just to be a citizen is unprecedented and unconstitutional. The likely way to legally force people to pay for health insurance would be for the government to tax people and then use the tax revenue to buy insurance.
In this tax and spend scenario the government would essentially control the money paid to insurance companies and this would give the government tremendous power over insurance companies. This power over insurance companies would give the government tremendous influence on the monies paid out for healthcare services. This would give the government a lot of power and control over healthcare providers.
There is nothing wrong with wanting to help people with pre-existing medical conditions and there are plenty of ways to do this. However, trying to help these people by the government forcing insurance companies to cover pre-existing medical conditions will naturally lead to government run healthcare.