When central banks cause bubbles, they leave a trail of broken dreams and broken bank accounts in their wake. Many of the newly rich find they are less rich than they thought, and many of those who bought in expecting to become rich find that not only are they not rich; they are much poorer than they were before. The bottom line: these bubbles induce people to speculate more recklesslessly and spend more in aggregate than they otherwise would have.
Then comes the crash. And what happens? Central banks cut interest rates, sometimes drastically. This induces those with any savings left to save less, and continues the dislocations caused by the original bubble.
And when some unsophisticate from outside the central bank priesthood points out that abnormally low interest rates might be harming saving (thereby limiting the pool of funds that would reliquify lending) and are thus keeping household balance sheets in peril, the reply comes that we can't hike interest rates now; after all, the economy is still recovering from the collapse of the bubble!
It is like saying to someone who's been on a merry-go-round too long and is now neckdeep in dizziness and nausea that he shouldn't get off the merry-go-round just yet. After all, he's not ready to be on his feet! Much better to stay on a lurching, spinning apparatus until he is no longer dizzy...