Why donate? Why wouldn't you invest in it and become part-owner yourself? But you can also flip that around: Let's say your dad opened a Facebook, but then die

Why donate? Why wouldn't you invest in it and become part-owner yourself? But you can also flip that around: Let's say your dad opened a Facebook, but then dies, why should you get $98 billion? You didn't do anything. Why is that now yours in the first place?