Because Medicare is funded 79% by income and payroll taxes and I was commenting on Medicare-for-all which is the most cited goal of single-payer advocates. If ACA policies were moved to Medicare, would not Medicaid recipients be moved to Medicare also? Then VA and private plans?
Federal entitlements are already the biggest driver of federal deficit spending, adding more recipients simply aggravates an already underfunded system. So additional funding is required by increased taxes or increased borrowing. Take your pick.