Austerity is as much the voters' addiction as it is the Tories and Labour have been framed as the "spend money for free stuff" party. Starmer has to combat that image if Labour want to succeed. No matter how many times you show the figures, people believe that Labour spends money and Tories save it. That's the kind of uphill battle Starmer faces in order to put Labour in power.
Note that we don't have a president. The Tories have changed leaders basically non-stop in recent times. If this doesn't prove to you that a PM is just a figurehead then I don't know what will convince you. Once Labour are in power, they can be held accountable on issues that matter.