Mostly because it adds cost and mechanical complexity for a feature most buyers don’t need often. The main purpose of a viewfinder eyepiece shutter is not to protect the glass, but to block stray light from entering through the viewfinder and affecting exposure during situations like long exposures when your eye isn’t covering it.

Some higher-end cameras have a manual viewfinder shutter for this reason, and others use a simple clip-on or rubber cover. Those solutions are cheaper, simpler, and reliable, so manufacturers generally prefer them over building an automatic internal mechanism.