Insufficient Airtight Seal
An insufficient airtight seal permits cold air from outside to pass through the insulation into the warm inside. The indoor air then dries out. This is caused by the fact that cold air cannot absorb as much water as warm air and it therefore has a lower absolute moisture content.
If this cold air is then heated, the resulting indoor air is very dry.
Houses with inadequate airtightness therefore tend towards a dry indoor atmosphere.

Example:
Outdoor air with a temperature of -10°C (14°F) for example, has typically a relative humidity of 80%.
Heating this air to +20°C (68°F) reduces the relative humidity to 9.9% (instead of 80%).
The more cold air enters the occupied room and warms up, the dryer the air becomes.

Even humidifiers can provide little assistance in this regard.