They don't trust people

That's why they're not saying that. They think that if they said, "Well, everyone should keep wearing masks in public for show, but we're pretty freaking sure vaccinated people can't really transmit it", that many vaccinated people would ditch the mask anyway and the anti-vax/anti-mask crowd would do it too and just lie.

It's the same reason people were lying about mask-wearing early on - everyone knew it was likely they were necessary, but because they didn't trust people not to buy all the supply up, they tried to dissuade people from doing it.

It's very frustrating and makes otherwise agreeable people wonder whether they're telling the whole truth at a given time.