Because as soon as it becomes clear that the central bank wants to achieve higher growth by letting inflation loose, this is incorporated into the exchange rate, price and wage decisions, which in turn leads to higher interest rates in the market.
Thus, monetary conditions are not effectively relaxed: we just end up in a situation that is much less predictable and much worse for growth. Consequently, monetary policy can stimulate growth only as long as it is certain not to jeopardise price stability.