It's just the cheapest one to set up. I was assuming that if someone's self-employed and not deducting insurance, they're probably a sole proprietorship, and are probably sensitive to accounting costs.
You can typically deduct 100% of your health insurance costs if you have a sole proprietorship. It doesn't matter how you're self-employed, as long as you have no other coverage and your business income funds the insurance, then you can deduct 100%.
No, that's one of the wrong assumptions / myths I'm pushing back against: LLC is _not_ necessarily the cheapest to set up. I'm the sole employee of my Massachusetts S-Corp, and it was both less expensive and better tax-advantaged.