Why don't we have cities with a massive subway system, apartment complexes a thousand stories high, and office complexes the same? So many people could live there...
Well, in the past, the issue has been transportation. Everyone needs food, water, and consumer goods to take in, but waste and heat must be removed. The average municipal water-grid only pumps the water to enough pressure to lift it up four floors. All of these mega-apartment buildings would need massive pump-rooms so that people higher up could still drink a glass of water and flush the toilet.
Beyond that, there are emergency evacuation issues. Should the building catch fire, how can everyone run to safety? Ladder over to the next building from the roof? Metal fire-escape stairs? Parachutes? A hang-gliding escape, while hilarious, might not work safely.
The last issue is that people probably won't like it. Experiments with rats showed that if you crowd them together too much, all the rats go insane. They attack each other. Their sexuality malfunctions. Their social cohesion completely goes out the window. I have no reason to believe human people would also have a limit, beyond which they suffer from want of privacy, and a crowded, crammed feeling.
If we keep expanding, we may have to take up such a mega-project. They're not making any more land, and no one is bothering with terraforming, space stations, or undersea biomes (hey, now there's an idea). If we don't keep expanding, though, the economy collapses, business suffers, and nations bemoan the dearth of citizens, as they do in many countries in Europe and Japan.