Because GDP measures only the quantity of money activity, and not its quality or usefulness, powerful banks and corporations enjoy big privileges.
Extracting and burning oil, gas and coal creates GDP, but no allowance is made for the human and environmental cost of the climate change this causes.
Using GDP as a measure, an extra dollar in a rich person’s bank account is worth the same as a dollar that buys a meal for someone who would otherwise starve.
Politics and money go together. When success is measured by GDP growth, governments are incentivised to favour the already-wealthy.
GDP feeds wars through the arms trade, encourages poverty through low-paid, exploitative employment and creates conditions for political oppression.
By using GDP as the measure of wealth, governments end up negotiating trade deals like TTIP that favour big, exploitative, multi-national companies.