Quantity theory of money:

MV=PQ

M is base money supply
V is base money turn over speed
P is price level
Q is GDP

This formula is proved to be correct even on a simple simulation of an island with only 2 people, so I trust it due to its simplicity

After financial crisis, central banks around the world increased the base money supply M by approximately 4 fold. But due to previous V is dramatically reduced, the result is that even GDP did not improve too much, P is relatively stable

In a fractional reserve banking system, V is tightly related to the money multiplier. If banks could actively loan out any money they have at hand, then V usually is 5 to 10. If economy is so bad that no one want to borrow money, then V will drop to 1, this seems to be the case now: M increased by 4x but V decreased by 4x, the result is that both GDP and price level stays stable

However, when economy eventually improved and loaning become active again, V will rise back to a normal level of 5, that will cause a 500% rise in price level if GDP keeps growing at a couple of percent per year, that will effectively crash the fiat money system

In such a case, central bank must quickly reduce the M accordingly. However, the way to reduce M is to sell their assets to commercial banks to get base money back, those assets include government bonds and mortgage backed securities, and a large sell of those assets will crash the market immediately and destroy many people's house value, drive economy back into recession

So it seems there is no way back once the hyperinflation is happening. The only way they can prevent a runaway inflation from happening is to let commercial banks hold their money and never loan them out. This means in a foreseeable future, there won't be large scale lending happening at commercial banks, that's the real reason that we didn't see any significant improve in economy, because from bank's point of view, the credibility of their money is much more important than economy