My windows face the street - and that's the issue, heat loss in winter is a big deal. And I have to actively monitor in order to know I should do it - this isn't something you really think about, and its terrible when you're in the flow while working.
There's also other reasons - for example I want to ensure the flow of air through the house concentrates and exits via the bathrooms and kitchen.
It isn't quite that easy. Even if there aren't impediments to opening a window a crack, it is not guaranteed that passive measures will reduce CO2 levels.
I've been tracking my home office's CO2 level for about a year and a half and obviously opening a window helps, but there are certain conditions (pressure, temperature, wind?) that require fans to normalize the levels.
The problem with ultra insulated homes are that C02 and moisture get trapped. So then you need filtered fresh air, and then heat recovery.