Because people are not nearly as rigid about a 9-5 as you are imagining in aggregate; enough people commute off hours to increase the number of round trips per car significantly.
Even in an office, there are plenty of people doing a 10-6, or an 8-3 then the school run... That's before you consider that many people are not office workers only going between the burbs and the city...
Though still, I wonder, what gains could be from such model (car sharing instead of cars or public transit)?
As far as I understand this model must support (with some room) demand at peak. So how many times less cars would be on the road then? How many streets and parking areas could be returned to people as pedestrian/recreation areas. How would that compare with proper public transport setup in terms of land use, cost, environment impact.
I don't think it would be that much, because I cannot imagine such car sharing model working in places like London, Tokyo or other big developed cities.
Even in an office, there are plenty of people doing a 10-6, or an 8-3 then the school run... That's before you consider that many people are not office workers only going between the burbs and the city...