Though Johnson's approval ratings have fallen since the start of lockdown (+32% in April, -1% so far in July), he still has the backing of 2019 CON voters (only 15% Disapprove). So there's not many CON to LAB switchers.
Starmer is an asset to LAB rather than a liability (Twitter would make you think otherwise). He has a net +20% approval compared to net ±0% approval for the Labour Party.
TLDR: Everyone is fairly popular so at the moment so nobody is changing their minds.