Or they do not know other options exist. Dont want to learn

Just Plan A and B are NOT enough.  This is common sense.

If you are relatively healthy, even though you have long term illness that needs maintenance care,  such as AF patients - Medicare Advantage Plan (Part C) should be enough.

If you unfortunately have some rare diseases or require very expensive drugs, you indeed need to buy Medigap to make sure you have proper care. 

Some biotech drugs are outrageously expensive.