Because my CO2/VOC levels are low enough that keeping in on all the time doesn't provide any additional benefit- while adding unwanted cool/hot air (no ERV is 100% efficient). The minimum airflow on my unit is 35 CFM (~60 m3/hr).
That makes sense. Though I imagine that depending on where your CO2 sensors are, it could take a while for higher concentrations to reach the sensor, and then more time for the concentrations to go down once the fan starts spinning.
One of the reasons why mine also runs when I'm away is that it helps prevent mould growth in the heat/moisture exchanger, as that thing is a bit of a pain to clean.
Also my ERV only runs on demand (~1/3 of the time, which is not enough filtration)