OnInvokeAsync for Action?

[seoyeon01]

I couldn't find it in the document,
It seems that using OnInvokeAsync allows UnityEvent to wait.

for example

public UnityEvent  TestEvent;
public UnityAction TestAction1;
public Action      TestAction2;

private async UniTask TEST()
{
    await TestEvent.OnInvokeAsync(this.GetCancellationTokenOnDestroy());
}

But UnityAction, Action doesn't seem to be able to use something like OnInvokeAsync
Isn't UnityEvent, UnityAction, and Action all public delegate after all?

Why can't await Action.OnInvokeAsync?