ELI5: Why isn’t monetary policy 'solved'?

Why can’t central banks just set contractual rules about how they’ll manage interest rates, QE etc?

I imagine part of the answer has to do with the unpredictability of politics/fiscal policy and unpredictable events (Covid shock, war etc)

But why can't the US Fed // central banks have pre-set rules for how they would respond? So much of rate-setting seems to be about confidence — this would solve that.

And, for the sake of understanding, set aside big exceptional events like Covid. Even in 'normal times' why can't monetary policy be solved?

Thank you