First, Medicare pays a lot more to healthcare providers than Medicaid. Medicare pays more for more medications than Medicaid, and has fewer prior authorization requirements for those medicines. Fewer providers will accept Medicaid, and people using Medicaid will receive less or worse healthcare than those in Medicare.

Second, Medicaid is administered by each state, and there is a lot of variability on how easy the state makes it so people can actually get healthcare. Lots of states straight up refuse money simply to punish people of a certain socioeconomic class because it happens to win votes.

Bottom line, Medicaid is so leaders can claim they are helping poor people get healthcare AND keep taxes low. Medicare is for actually delivering healthcare to people because that contingent makes up a huge proportion of votes.

And yes, even Medicare is not delivering all healthcare, as it has multiple tiers to deliver differing amounts of healthcare to different socioeconomic classes.