Because purchasing power is quite hard to quantify. It requires you to define a set of goods the "average" consumer uses. This leaves much more wiggle room for manipulation and politicization. [1]

Wage in currency is a lot simpler. The only real manipulation possibility is currency value and some minor tinkering in the definitions.

[1] For example a major criticisms of the euro-zone inflation index is that it excludes housing cost: https://en.wikipedia.org/wiki/Harmonised_Index_of_Consumer_P...