This crosses into "can't prove a negative" territory - we can establish how long the virus can be detected on surfaces, and we can establish how long it remains viable enough to reproduce in cultures under lab conditions, but there is no ethical way to prove or disprove whether those samples on surfaces are adequate to cause infection because that would involve exposing healthy people to the virus. So the best we can do is go by the real-world data on spread, as established by those jurisdictions that are doing thorough contact tracing... and that data shows an absence of new infections that are not traceable to other cases.
As far as "why wash/sanitize?", well, unlikely isn't the same as impossible and good hygiene during a pandemic is just common sense. It is a 100% painless way to err on the side of caution, so what sense would there be in not making that recommendation?