There are back-up flight attendants at the airport
When passengers see a flight attendant rushing to get on to the plane, they aren't usually late but covering for another cabin-crew member.
In case a crew member can't make it to work, there are one or two standby flight attendants ready and waiting to step into their colleague's place at the airport, with their uniform on and their luggage packed.
"I have been called to work a flight because the flight attendant who was supposed to work it got into a bad car accident on his way to work," an unnamed flight attendant wrote. "I got to the gate within 5 minutes of my notification and met with glares from the passengers."