Because your premise is false.
"They" are not buying and selling gold.
What you name "premise" here, is not "mine", it's data from gold.org that shows annual net purchases or sales.
You can create a free account, after which they show that, and other data.
I created an account in 2011.
They show data from 1997 onwards.
Worth noting: data can get revised in later years. Due to CB's trading but publishing only years later.
This was the data I accumulated in 2018:
1997 326 $330.98
1998 363 $294.24
1999 477 $278.88
2000 479 $279.11
2001 520 $271.04
2002 547 $309.73
2003 620 $363.38
2004 479 $409.72
2005 663 $444.74
2006 365 $603.46
2007 484 $695.39
2008 235 $871.96
2009 34 $972.35
>>> 5592 tonnes gold sold over the period 1997-2009
2010 -77 > -79.2(2016Q4) $1224.53
2011 -455 > -480.8(2016Q4) $1571.52
2012 -544.1 > -569.3(2016Q4) $1668.98
2013 -386.6(2014Q1) > -409.3(2014Q2) > -625.5 (2015Q1) > -623.8(2016Q4) $1411.23
2014 -477.2(2014Q4) > -588.0(2015Q1) > -590.5 (2015Q2) > -583.9 (2015Q4) $1211.71
2015 -588.4 (2015Q4) > -566.3 (2016Q1) > -576.5(2016Q4) $1160.06
2016 -383.6 > -389.8 $1250.74
2017 -371.4 > -374.8 (2019Q1) $1257.12
2018 -651.5
>>> 3674.7 tonnes gold bought over the period 2010-2017
Those lines <year> with several figures tonnes purchased, separated by > marks, list the revisions.
"2014Q1" refers to a gold.org quarterly report, where the revision appeared.
Yet, you're stuck in some denial mode, your choice...