It doesn't prevent the person from obtaining health insurance. It imposes a waiting period for claims against that condition. The waiting period can vary from policy to policy. Claims for diagnoses unrelated to the preexisting condition are covered.
I was referring to pre-Obamacare regulations.
In numerous states no legislation existed to force insurers to accept applicants with pre-existing conditions, waiting period or not, and they didn’t.
Over the years I have corresponded with any number of individuals who could not purchase health insurance at any price, to cover any condition, because of pre-existing conditions.
My own state, Washington, had a nine-month waiting period but even then, insurers could reject an applicant and send him to the state high-risk pool (which was sometimes not accepting new individuals).