The only plausible reason I’ve heard to be “nice” to LLMs/virtual assistants etc. is if you are being observed by a child or someone else impressionable.

Very much this but, we're all impressionable. Being abusive to a machine that's good at tricking our brains into thinking that it's conscious is conditioning oneself to be abusive, period. You see this also in online gaming - every person that I have encountered who is abusive to randos in a match on the Internet has problematic behavior in person.

It's literally just conditioning; making things adjacent to abusing other humans comfortable and normalizing them makes abusing humans less uncomfortable.