If there is no other source of truth it's easier for the government to change the data rather than the actual air quality.
Everyone and their dog has air purifiers that measure AQI and PM2.5 in Chinese cities now.
They already do, in a sense. The scale used here is different from the internationally used and the categories imply lesser health effects.
To be slightly more specific, Chinese authorities consider PM2.5 levels of ≤70 mcg/m³ as safe, while by EU standards that same pollution level is considered "extreme".
The WHO yearly recommended limit is 5 mcg/m³, by the way.
To give you an idea of the difference in perception. When we lived in Beijing, anything below 150 mcg/m3 was a "good day", <50 was "amazing". We didn't start taking any precautions like closing windows until it was >250. We stayed indoors when it was >400 or >500.