Why doesn’t the Toyota Pious have plug-in capability?
Because like that EV evangelist Bob guy said the secret of long battery life is to not use them.
From what I read the Pious only uses the middle 40% or so of its battery capacity. It doesn't charge the batteries above 60% and won't fully discharge them without pressing the EV button which American models don't (or perhaps didn't) have.
If it had a mains socket owners would want and expect to be able to deep cycle the batteries daily and end up having to replace them every 2-3 years.