One problem is that, under current law, you tend to only get those rates until you get sick. Insurance companies aren't allowed to actually jack up your rates if you get sick, but they tend to do so in tranches: every N years they'll discontinue one insurance program and institute a new one. Healthy people can apply for the new one with lower rates, but people with preexisting conditions will be denied a transfer to the new program. The old program will then enter a death spiral where only people who have e.g. cancer or diabetes are stuck there, because once you're seriously ill you can't go anywhere else (at least as an individual purchaser). Then the rates start being jacked up each year to account for the now-less-healthy pool. (This pattern is called the "closed block" problem, and some states have been attempting to add new regulations to restrict it.)

And of course if you had a childhood condition (beat childhood cancer, congenital heart defect, etc.) you can never buy in in the first place.