Imagine if you could buy car insurance after you crash your car.
Huh? The "car crash" in this analogy is "losing your job", which has nothing to do with your health profile.
And in that circumstance you are allowed to maintain your health insurance (COBRA) or buy a new plan ("qualifying life events," which also includes things like marriage and moving).
The comment you're responding to was alluding to if people could choose to not pay for health insurance until after they got injured or sick and then needed the benefits.