To be fair, maybe in a post scarcity space faring society that land is abundant on Earth as most humans live elsewhere now, allowing the Earth to heal ecologically. Perhaps he only "owns" it insofar as his position as a starship Captain for the Federation allows him to have this space to himself because no one else would use it. Perhaps the "servants" actually live in large nearby houses and this is just what they wanted to do with their lives and how they wanted to live and since they take care of all the properties anyway, why not upkeep the small portion of the property that a Captain actually lives in.

I dunno, just spitballing. Probably wrong on Earth being less populated somehow. I'm not queen of star trek trivia or anything.