Why are people instructed to wear masks once fully vaccinated? at that point, isnt the risk the same as flu?

There are many reasons to continue wearing masks. A fraction of those vaccinated will not gain immunity. The emerging variants may not be prevented by available vaccines. I wear mine out of respect for those who died & those who have not yet received the vaccine. I wear it in public to acknowledge that the disease is worthy of respect as are our neighbors. At some point we can stop.