The lifespan of a house is a ship of theseus type question. The wiring in the walls, plumbing, foundation and frame could last 50+ years if everything else was done right, and there is no water going where it is not supposed to. Modern roofs, windows, air conditioner and heater last 25-30 years. A water heater is going to last about 15 years. Exterior paint, interior paint, carpet, all last about 10 years. After 50 years you will have replaced a substantial portion of the value of a house even if it looks more or less the same.

Even if the home is habitable there are other reasons they are removed. After a certain point it makes more sense financially to tear a home down and rebuild than to continue to renovate it. Also markets change over time, and homes are not always built in a way that maximizes the use of the land. An area that was mostly bungalows and ranches can have homes replaced with larger homes or multifamily buildings.