Economists call this underlying volatility “shocks” – unexplained shifts in consumer demand or business behaviour, or oil-price shocks, or shocks from foreign economies, and, yes, financial market shocks. This volatility must go somewhere in the economy. Central banks target inflation, which means stabilizing inflation and economic growth, not interest rates.
Pursuing that mandate means that financial markets carry more of the natural volatility that arises. To make this point in its most extreme form, consider how a hypothetical, all-seeing central bank would behave. That central bank would anticipate all shocks to the economy and move interest rates up and down to offset them, so inflation would always stay right on target.