Couple of points:
Yes, generally Medicare is open to all seniors 65 or older long as they meet certain qualifications for eligibility. However those who have not paid into the system (or a qualifying spouse/former spouse) must pay insurance premiums for Part A. Those who qualify do not, thus Medicare isn't exactly "free" for all seniors. Granted most persons who have reached 65 and were born in USA should have ten years cumulative of payments into Medicare.
https://en.wikipedia.org/wiki/Medica...s)#Eligibility
Rest is as everyone has stated; Medicare does *not* cover everything, hence need for various supplement plans unless senior in question can afford to self fund the balance of their healthcare needs.
Those who are poor enough to qualify for Medicaid can receive those benefits along with Medicare. The former in some ways replacing what a supplement plan would have provided.
Sadly many Americans do not bother educating themselves about Medicare (or Social Security) until they approach qualifying retirement age, and or actually at that time. Many then often get the shock of their lives when are sat down and some home truths are explained.
People do not know/realize that Medicare premiums are taken off the top from their SS checks.