Because we live in a multi-currency world

Scot Sumner touting his NGDP targeting scheme:

Why not both?

So why not have the Fed set a 4% NGDP target, level targeting, and offer to sell unlimited NGDP futures at 5% and buy unlimited NGDP futures at 3%? In that case, the rule would be that the Fed is forced to put its money where its mouth is, anytime their policy views sharply diverged from the market consensus on expected NGDP growth. As a practical matter, the huge mistake of late 2008 would have been impossible under my "guardrails" approach because investors like me would have gone short NGDP futures at the 3% price, as it was very clear we were going to have sub-3% NGDP growth in 2009. (NGDP actually fell by roughly 3% from mid-2008 to mid-2009.) If the Fed didn't respond adequately, their losses would have been massive.

The dollar is dominant because it is the  only tax extinguisher, so Scott's NGDP targeting scheme will be used to hedge future taxes against inflation.  The finance industry has the technology to keep liquidity events hidden from the Fed. at least until taxes are reduced..