Ehrman and Joseph of Arimathea

Ehrman states that because Paul doesn't mention about Joseph of Arimathea, it must be because he doesn't know anything about him burying Jesus. One argument from a website against this is that because Peter was Jesus's top disciple and James was Jesus's brother, they would have very likely known about who buried him. Because Paul worked with them both, he would have known from them. Problem with that argument though is that I myself don't know the name of who cremated my own father even though I was close to him. If many people don't know the name of who cremated or buried their relatives, why should it necessarily be the case that Peter and James would have known?