The door in use in your photo is marked as a fire exit. If you look closely, you'll see the larger panel the fire door is contained in, as well as the almost identically sized panel to the right of it are both actually gates. (You can make out the hinge connection at the top right). THis is also backed up by the vacant space in front of them. If those gates aren't in use regularly for vehicles, there'd be no reason to leave the space in front of them clear to swing them open.
Note also, just above the left side of the gates, is a large number of stacked pallets. You don't do that by hand - thats a forklift, and that's a vehicle.
This is also supported by the yellow lines painted on the ground - a good WH&S indication of 'keep clear' or 'pedestrian walkways' (go to bunnings - you'll see them).
I have to disagree... sort of.
I guess, yes, they could better use their land if they tetris-ed everything in - but Luna is a little more about the whimsy, not the regiment. They're never going to be doing any sort of large expansion. I'd rather not start clusterfucking the park for the sakes of fitting in an extra flat.
As for the grassed area, I feel like the roof of the carpark probably wasn't built to withstand weight on top of it, or at least more than what is on it. The carpark would probably need significant reinforcing works to have large moving machinery plonked down on top of it, with is probably prohibitive. In addition, the need to put in a ramp or accessway to get to it would also take up more 'in park' space as well - which could probably be better spent used for other things.