> Banks have absolutely no incentive to provide money to everyone that wants to borrow all their future life earnings now, not because of hard limits on their funds but because they want their lending to be repaid at a profit (when it won't be, you get 2008).

2008 happened because clearly the banks do have that incentive. The only thing that stops them are the regulations.

> The "reserve requirement" (technically replaced by a capital requirement) isn't something central banks tinker with,

In the last two decades that requirement was adjusted at least 5 times in my country so tinkering with it is definitely a tool that some central banks use. In the nineties it was even set to 30% to quench hyperinflation.

> Central banks were created to stop banks with solvent loan portfolios collapsing due to demands on their reserves, by ensuring banks could always borrow the reserves to back up the numbers on their spreadsheet.

First formal central bank, The Bank of England was created to finance the war. Central banks gradually acquired their modern roles as they developed.

The role you so much focus on is called being 'lender of last resort' to private banks. Private banks use it only if they can't get money cheaper anywhere.

> they influence credit action by adjusting the price of borrowing those reserves, and thus the demand for credit in the wider economy.

That the thing they do most often but not their most powerful tool.