Why does the level of debt mean the government "has to lever in private finances"?
Any private investment will require a return which is a public cost.
The cheapest return for the government to pay out is on debt.
Any other investment comes with the illusion of the private sector bearing risk, which they want paid for. They never bear that risk though. It's underwritten by the government, often tacitly.
Also, investment buys assets. If the government has assets it either spends less renting them (eg hospitals, schools) or gets income from them (housing, utilities)
Just because some gobshites on the telly say that government debt has to come down, doesn't make it so. "spooking the bond markets" and "maxing the credit card" type bollocks is just bollocks and bears no relation to reality.