If we did away with the pre-existing condition rule, then insurance companies could freely discriminate, but then millions of people who need health insurance would be at risk of losing it. (blog.politics.ox.ac.uk)
You'd owe the least if you didn't need any health care all year long. (verywell.com)
All that, and it doesn't help that the people who are enrolling aren't necessarily the people the insurers want (i.e., the ones who need health insurance the most). (policygenius.com)