With probabilistic algorithms, you still have the fundamental limitation that there's a lower-bound to how fast an algorithm can run. You can't keep getting faster indefinitely.
That isn't possible, but it might not quite be obvious why that isn't possible.
To have sub-constant time it would have to go to 0 as n increases, as otherwise we could bound the time below by a constant. This isn't possible, as a computer will take at least 1 time unit on every input. So the expected time will always be at least 1.