"Leaving" can be a long, drawn-out process.
Once they put themselves on the "emigration" list, the persecution comes fast and hard.
You need someplace to travel to, even if you leave all of your possessions behind and get airline tickets, there is no guarantee the nation you're travelling to will let you stay, especially if they don't want to piss off the South African government, ("or are you a racist"?)
I think those that have stayed to date have decided that SA is their home and they are not leaving. The Boers have been there hundreds of years. They will take the consequences.