The United States primarily relies on employers to provide health insurance coverage to their employees and dependents. There is no universal healthcare (government programs are confined to older persons, people with disabilities, low-income mothers...
Because of the saving advantages of an HSA, and its ability to rollover from year to year, it's a good idea to max out the contribution limits if you're able.
In simple terms, it's proof of good health. An EOI application requires review and approval by the carrier before certain life and/or disability insurance policies become effective.