Like, central ventilation is not magical unobtainable technology. Simple heat recovery even vastly improves heating costs in a way insulation never can.
What of the buildings that don’t comply in time? Or can’t find trades to do it in time? Or we notice to our eternal shock that projects to ensure code tracking are priced at a serious premium?
Or, how many improvements to code would we decide weren’t desirable because of the costs of retrofitting, so now we lose even the low slope of improvement versus today.
Not to mention, a lot of places around the world care about the look and character of historical locations. If a structure wasn't designed for central HVAC, for example, then there's often nowhere to hide the condenser units, air handle units or the ductwork. Same with insulation -- if that exterior wall wasn't intended to have it you've got a couple options and they both hurt.
Last of all, I'll mention labor. The type of skilled labor that can do any specific trade at all is relatively rare in the aftermath of the "college-or-bust" era, but the kind of labor you'd want for renovation work (fast, efficient, can tackle multiple different aspects at once without calling in different trades, and gets it right the first time to minimize disruption/call backs) is even more rare. To carry out some kind of massive renovation project at a national level even with infinite money you're talking about a generational timeframe just due to labor constraints.
There is a lot of old buildings out there.
Not everyone lives in a disposable McMansion in a gated community.
These buildings will have ventilation eventually but the code needs about 20 years to take full effect.
Mind you that people have been living in those buildings for hundreds of years without issues but tennants these days don't seem to have the basic skill of airing their place.