AJ Riley
It is simply because no one has added them to the map yet. If you're new to OpenStreetMap, check out the help page ( https://www.openstreetmap.org/help ) for an introduction.

If you're interested in helping out, there are a couple ways to map houses. If you know the address of the house, you could simply add a single node to where the house is and add the relevant address information to it. Alternatively if you just want to add the outline of houses to the map, you could trace them out in the editor using the provided imagery.

If you have more questions, feel free to ask.