And what would these people do? Floating weightless on a space station your entire life with absolutely nothing to do is not an existence we should be engineering. You'd also be suspending a quadrillion people in a completely dependent situation, constantly one supply ship away from disaster and saddling the rest of humanity with supplying them. Half the point of colonizing Mars is to create another space habitable by humans, not merely a habitation.

And if all you want is mass, we have many asteroids not stuck at the bottom of a huge gravity well to mine.