I think they would like to pick up the car, drive somewhere else, and then leave the car parked somewhere for a weekend without paying for it in that time. Which is difficult to make work as a business (is anyone else going to rent it in that time? Would they be happy if all the cars were to be driven away somewhere else?) Even more traditional rental companies will often charge you extra to drop off a car one of their locations which isn't the one you picked it up from.

Zipcar and co are generally aimed at daytrips, the kind of thing where you occasionally need a car or van for a day but no longer, not for longer distance trips where you are away for a few days.