Duh!

As has been pointed out, the reason that central banks pumping money into the banking system for months has done nothing is that the conceptual model is backasswards.

The usual explanation is that the central bank "prints money" by lowering the rate at which it lends. Money then moves into the economy. This is true, but only sometimes.

What has happened over the last 20 years is an explosion of consumer credit, so the consumer could now also "will money into existence" by borrowing from a lender.
You go to a store to buy a giant plasma TV on your credit card... poof! 5,000 dollars comes into existence.
You get a car loan... poof! 25,000 dollars comes into existence.
You take out a second mortgage (euphemistically called a home equity loan), and poof! 50,000 dollars comes into existence.
So, previously, central banks were the only ones who could will money into existence (especially after they dumped the gold standard), but now, everyone could do it! And so they did.

This has now come to an end and will not be revived for a long time, perhaps decades. That is why the only thing left is for central banks to go to zero interest rate. And it will not work. And it could, sooner or later, cause horrendous inflation. This is difficult to foresee because timing is nearly everything, and the actions to be taken by governments are unknown.

If we are lucky, we will have a worldwide depression for a year or two; if we are not, for a decade or more.