Are these listings for apartment complex units for rent? If so, are the listings posted by a real estate agent, NOT the complex management?
Then it may be that the agent does not have personal photos of the interior and there are copyright regulations about using photos of others. The agent is offering to help you lease an apartment there, but does not necessarily have the complex management's permission to use existing property photos.
If it is a privately owned property where the available unit is still occupied, then it may be as you suggest, the tenant does not want photos of personal belongings made public.
I agree w/you that is would be more helpful to see photos of the space available, a floor plan...something other than the pool, parking lot, etc.