Why the fuck does Starfeet even bother building starships then?
Because transporters don't help you keep the balance or fight off the Klingons, Romulans or whoever else, when they come for you with their fleet.
And because you need starships and their facilities to explore unknown regions of space. You can't keep beaming people blindly hoping to hit a planet.
Then why didn't Admiral Robocop just beam a bunch of nukes to all the Klingon planets? How could they prove who did it?