Condensation, film, and what interior glass actually collects
Interior glass does not get dirty the way exterior glass does. It collects a film rather than a deposit, and the film has three sources in an Omaha home.
Cooking is the main one, particularly in open-plan houses where the kitchen shares air with the living space. Airborne cooking oil settles on every cold surface, and glass is the coldest surface in the room for six months of the year. It builds slowly and evenly, which is exactly why nobody notices it happening.
Then there is off-gassing from soft furnishings and new carpet, which leaves a faint haze, and in homes with a woodburner or a gas fire, combustion residue. Add condensation cycles through a Nebraska winter, and you get a film that has been wetted and dried a hundred times.
That film does not come off with glass cleaner and paper towel. It smears, because you are redistributing oil rather than removing it. It needs a detergent solution and a proper wet-then-squeegee pass, which is why people who clean their own windows are often surprised at how much clearer the room gets.