Physical area we could potentially pack people into isn't really the problem. There are a lot of other considerations like biocapacity and employment options which are also important concerns. We could put a lot of people in remote parts of the world but where will they work? Shop? Those places often lack the kinds of critical infrastructure that would attract and sustain a thriving population.

I think overpopulation isn't our most pressing issue right now, but generally there are a lot of advantages to having fewer people.

They lack infrastructure because there isn't the demand for infrastructure. We build where there is demand, and the building creates further demand. It's a self-reinforcing cycle.