Growth. If we get the economy growing, debt falls as a percentage of GDP. Markets feel more confident that the economy is on the right track and bond rates and government borrowing costs fall, releasing more money to spend on services. And in time, inflation devalues our debt even further. EVERYTHING the government does should be looked at from the perspective of how can we increase growth (and productivity, which drives growth).
And why can we not just "tax the rich"? Because theres not enough of them and they are mobile and flexible enough in their tax affairs to avoid paying it. It's not as simple as suggesting Haaland wouldn't leave. Maybe the next Haaland wouldn't sign if the income tax rate was higher? The very rich have houses all over the world and perhaps stay in London for only 6 months per year. So they decide to only stay 179 days instead, to avoid UK income tax altogether perhaps? Or they move their businesses' HQ to the Netherlands and make their profits there whilst continuing operations in the UK? There's all sorts of ways rich people will avoid paying tax and the higher the rates, the more they avoid it.
Look at the converse. Lower tax rates and look at how many global companies like to put an HQ in Ireland. Apple, Google, Facebook, Microsoft, Novartis, Johnson & Johnson, Intel etc etc. Why are these businesses in Ireland, generating billions for the Irish economy, and not in the UK? Because of a favourable tax regime. High tax rates drive tax revenues away. Low rates attract them.
This is not rocket science. It's human nature and plain common sense. People don't like being stung and will try to avoid it. If we want maximum tax receipts we cannot just try to sting people.