Intercst is right. People want to keep all their unhealthy habits and take a pill to fix everything.
Supplementation can easily fix Vitamin D3, selenium, and iodine deficiencies. No massive overhaul of the dysfunctional food system is required. However, most people aren’t taking these supplements.
Just one Brazil nut per day provides ample selenium. Cheap selenium supplements exist for those who lack access to Brazil nuts, have a nut allergy or sensitivity, or have simply run out of Brazil nuts.
I take a kelp supplement, because very few foods have it.
I take a Vitamin D3 supplement, because foods have far too little to matter, and it’s not always possible to get Vitamin D from the sun. The conditions that make it possible to get Vitamin D from the sun also make it possible to get sunburned or develop skin cancer. I used to skip the Vitamin D3 supplementation in summer, but not anymore. Relying on the sun for Vitamin D would mean having to thread the needle between too little and too much.