For the same reason that cold air seems to blow in through doors and windows on a still winter day.
Convection
The inside of your house isn’t the same temperature as the outside and therefore the differing densities of the inner and outer air will begin to move in an attempt to settle out with the cool air on the bottom and the warm air on top.
If you have access to an IR camera this is a really neat effect that’s visible on the walls when the different temperature air begins to stratify inside the house.
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