Pre-existing Conditions

I just read in an opinion piece that denying health insurance to people with pre-existing conditions was one of the insurance industry's "most abusive practices."  I wonder why fire insurers won't cover you if your house is on fire.  The author of the piece admits that insurers could only do this if everyone is forced to buy coverage.  I wonder why insurance companies are blamed for not being a charity.  If they are so bad, why not do away with them?