Group health insurance through employment does cover pre-existing conditions.
I think the biggest problem with pre-existing for just anyone is people gaming the system and not just the cost to cover. People can cruise along opting to not buy insurance and save their money, then they come down with something and immediately go out and pick up coverage after finding out they have cancer. WHo wouldn't do this? So they require you change jobs or get a job with a plan (which often have a waiting period to verify you're really there for the job vs. getting a procedure done) to cut down on this.
Think about it... it's a rock and a hard place and not a one sided arguement. There's no perfect health system on this planet, they all have their problems to be worked out.