Because I hadn't thought about that before, but it is actually quite a concerning identity - eg, looking at the price of gold, clearly asset prices are rising in response to increases in the money supply. A kilo of gold in 2017 has nearly the same utility as in 2000, but there is a substantial price rise.
Since inflation doesn't measure things like that (only measuring changes in consumer goods EDIT - [1]) - wouldn't a substantial portion of the growth in GDP then be a response to the fact that the M2 is rising and there is trade in goods that are not included in the inflation indicies? Basically, if the central banks were forcing growth in the M2 (which they are), wouldn't that call into question the value of the real GDP as a measure of real economic growth? All it would take is that price rises in assets or business costs, caused by increases in the money supply, not be at the same percentage rate as price rises in consumer goods.
EDIT - [1] - Obviously you can get the real price of gold after adjusting for inflation, but that is assuming that the impact of inflation on the price of gold is identical to the impact of inflation on the price of consumer goods. I hadn't realised that assumption was being made before, and I'm going to read up on it to see what evidence there is that it is true, because if the impact of monetary supply increases on business and asset prices is different, then that undermines GDP growth as a meaningful measure. Thanks for the thought, it is an interesting one.