Because that is the way the tax code is written.
If you pay health insurance premiums with out of pocket funds that were already taxed then you can enter premium payments as an itemized medical deduction. Those that did not pay their health insurance premiums with income that was not already taxed cannot enter those premiums paid as an itemized medical deduction.
Health care insurance premiums, including dental and vision insurance premiums, and other medical expenses that you paid with out of pocket funds and were not reimbursed are an eligible medical expense that you can deduct using Schedule A for itemized deductions. However, only your total medical expenses that are greater than 7.5% of your Adjusted Gross Income (AGI) can be deducted. Your total itemized deductions reported on Form 1040 Schedule A must be greater than the standard deduction for your filing status to have any tax benefit.
Standard deductions for 2023
Single - $13,850 add $1,850 if age 65 or older
Married Filing Separately - $13,850 add $1,500 if age 65 or older
Married Filing Jointly - $27,700 add $1,500 for each spouse age 65 or older
Head of Household - $20,800 add $1,850 if age 65 or older
