I don't disagree, but Labour received far more votes (and members) with a left-wing manifesto than Starmer received at the last election, while economically left-wing policies like wealth taxes and the renationalisation of failed privatisations *are* hugely popular with the electorate.
In the run-up to Trump winning his fist election in the US, Bernie Sanders was polling higher than Clinton (and beating Trump) on an economically left-wing platform.