In the middle of a national crisis he has no choice but to let the Tories control the narrative to a major degree. Any attempts to change the topic is completely unacceptable and if he engages in too overt an attack on the government at a time when they're still dealing with the pandemic is going to lead to a big backlash against Labour for 'putting politics above the national interest'. Starmer seems to be doing an excellent job so far of holding Boris' feet to the fire at PMQ's while projecting a calm, competent image to the country that contrasts well to Boris and his bumbling and dishonesty.
Restoring Labour to electability is going to take time, and anyone thinking he should be making radical steps right now is not thinking things all the way through. He's building a profile of a realistic Prime Ministerial candidate, letting the Tories have enough rope to hang themselves, and taking steps to remove Labour's rough edges and contentious positions. By the time this crisis ends they'll be seen as a serious party who can start laying out their own platform while stomping the Tories over their Corona failures and likely the failure of the Brexit strategy as it comes back to the forefront.
I really don't get this insistence on attacking Starmer from within right now, he's doing a good job in very difficult circumstances.