Why are "ground level" air quality monitors really on rooftops?
In order to put your question in perspective, you need to consider atmospheric mixing and the size of the system you are curious about.  The troposphere is several kilometers thick and the mixed layer of atmosphere above the ground can occupy a significant fraction of the troposphere.  Granted, during cold events, the temperature of the air can be so cold that the lack of convective motion creates a very shallow (e.g. 10s of meters) mixing layer.  Though, 15 meters above the ground will virtually always be within the mixed layer near the ground, despite cold weather events.

Some people do measure air quality at the ground level, but it is generally done in the context of studying biogenic emissions (e.g. soil emissions or below-canopy tree emissions).  However, those are special field studies, which have different objectives than typical air quality monitor sites.  

Typical air quality monitor sites are relatively sparse, and even large cities have just a few monitors.  Generally, monitors are located in such a way that they are observing well mixed air, so that it represents air quality in the region.  It is imperative that you put your air quality monitor high enough above the ground so that it isn't being directly influenced by emission sources (e.g. your neighbors wood stove, or a car's tailpipe).  Otherwise, if you put the monitor literally on the ground, you will often be observing much more variance, with higher peaks that represent that specific location only.  See this post for more info on how monitor locations are selected.  You can also see this EPA handbook on siting monitors which notes: