Comment by amluto
Competently built modern houses are well sealed, so not much air filters in through leaked walls, doors, etc. Instead, outside air is actively introduced through a filtered intake, by a fan (or by deliberate negative pressure, but that’s riskier, as air will come in through other paths too, bypassing the filter and potentially introducing contaminants from the structure and/or soil into the building).
There are plenty of systems to do this. My favorite is an ERV, with an aftermarket, oversized, upgraded supply filter.
While it's totally possible to build competently, my impression is that, in the US at least, there are tons of existing houses—built roughly between the 70s (energy crisis) and the mid-2000s when ventilation requirements became more common in building codes—that are fairly tightly sealed but lack any sort of real ventilation systems beyond like, kitchen and bathroom exhaust fans.
Having grown up and lived primarily in (uncomfortably!) drafty old houses, I've noticed the phenomenon ever since I was little kid because they have a distinctive stuffiness and smell of furniture/carpet/plywood even if nothing inside is actually new. I think many people are just used to it/consider it a normal smell of a house because so much of the housing stock is in this category.