CB's just kept repurchasing until unemployment bottomed out, and now that we've hit the bottom, they will start to shrink their balance sheets. That's their mandate - maximize employment, keep consumer prices relatively stable.

All CBs can really do is make sure banks have enough money to lend into the economy by adjusting reserve requirements and interest rates. They could massively screw up the economy by jamming interest rates up right now, or dumping all their purchased QE assets back on the market, but they can't really improve the economy strictly through monetary policy.

That would require better fiscal and regulatory policy, such as tax code reform, increased spending on R&D and infrastructure, spend on education, etc. Even then it's not guaranteed this spending would lead to any technology-advancing breakthroughs that could raise the standard of living across the economy.

Ultimately I believe that low productivity (we've hit a ceiling on the returns to the internet, computer, and smartphone for the moment), plus a lot of the rest of the world catching up in terms of infrastructure, is to blame for the stagnation.