Because by publishing & using true data, the Bureau of Labor Statistics, Labor Department & Federal Reserve would wreak havoc across financial markets and the wider economy – something that's inevitable anyway, but why not 'guide' perception alchemically while continuing to kick cans down the road?

Imagine if the real inflation rate of 6% were listed?  Would holding US government bonds, which pay around 2%, then make sense for you, the personal investor, let alone for foreign central banks or institutions which recycle oil revenue receipts into US Treasurys?  And wouldn't the government then need to pay more for CPI-linked entitlement payments like Social Security while corporations would be forced to raise wages?  Most critically, wouldn't interest rates then need to be hiked by necessity, rather than remain as 'merely' one of seemingly many options available to clearly policy-cornered global central banks?

You start seeing the massive stakes underwriting requisite lies, and thus how & why everyone dances around numerical dead elephants in the living room.  Yet mathematical inertia tends to do its own thing, so best to be properly informed and plan around the government's window dressings.  I.E. Imagine that it's early 2007 and you knew then what you know now.