The problem is that the UK resoundingly voted against the opportunity to elect Corbyn as PM on account, it seems, that he was considered too far to the Left. On top of that, we’ve had 14 years of the Tories who essentially won the last election on Brexit and its position on immigration and has lurched even further to the Right in an attempt to shore up its vote. That should tell you something about how far the public would be inclined to vote on another hard Left candidate for PM.
I don’t know whether Starmer is actually a centrist, but it seems clear that the country is not anywhere near the far Left, so it only makes sense to position yourself in the centre and seek to bring the public along step by step.
It’s now 8 years since the country voted in favour of Brexit and only now are people prepared to speak up about the disaster it’s been after 8 years of the Voldemort effect.
The hard left had its opportunity and the public voted in the other direction. It doesn’t make any sense to pine for a hard left politician when the public won’t vote for it.