Could not check in because the flight was "over booked"

On Friday the 14th of June 2019, I arrived at OR Tambo airport to check in for the 18:55 SAA flight to East London. The person that was checking me in told me that he can’t check me in because there are not seats available on the flight. I asked him how is that possible and he told me that SAA tends to overbook flights. He then tried to get me a seat on another airline, but he was unsuccessful. He told me the only flight that is available is the 10 am flight but that flight is also full so I would be put on standby. I told him that I cannot miss the 18:55 flight because I am attending a funeral on Saturday morning so they have to make a plan. He then printed my boarding ticket and asked one of his colleagues to escort me to the boarding gate and tell the boarding staff that I am on standby and they should check me in once a seat is available. I then sat at the boarding gate and waited for everyone to check in, I then approached the boarding staff. 1 of the boarding staffs went onto the plane to check if there are any open seats, she came back and told her colleague that there are 2 open seats. Her male colleague then called someone and asked them to load my name onto the system as a passenger, the lady on the other line kept on saying that she can’t load me. Then the lady that had acquired the 2 seats said she will try load my name, she kept on mentioning that the flight had been delayed for too long and they want to leave. I then pleaded with her to load my name because I can’t miss the flight, while she was loading my name the flight attendants closed the door and prepared for take-off, as a result I could not board the flight.

I am very hurt and disappointed by the lack of empathy.