system
Given the following post, respond as if you're commenting on Reddit:
human
[Blade Runner/sci-fi in general] How/why do entire cities like Los Angeles get completely rebuilt in a relatively short time?

The original Blade Runner in 2019 features a massive Los Angeles with all sorts of all-encompassing architecture of impressive size. When we compare it to 2019 of today, the change is nowhere near as drastic as it was in 1982. This is common in a lot of science fiction; in just a few decades, these major cities are completely redesigned and unrecognizable.

What prompts such a massive rebuild? I know the Blade Runner universe was ravaged by nuclear war, but there are still remnants of the old city like the Bradbury Building, implying LA was largely untouched by it. Urban planning isn't cheap; even with advanced technology, how would it be feasible to completely restructure the city's layout, and what purpose does it serve? What is different in the timeline of Blade Runner (and other similar sci-fi) to make this happen?
gpt
Massive amounts of people came into L.A from other cities.

For instance, Vegas is completely deserted after being hit by a dirty bomb, and San Diego is in ruins, with shamrock factories and homeless people who formed militias.

Most of the people moved into L.A since it was untouched by the nuclear war, at least those who couldn't afford a ticket to go Off-World.

The sequel leaves L.A in an even worse shape; while, before 2019, L.A's population was stable, the Blackout and subsequent ban on Replicants worsened the situation. Off-world colonies suffered, and as such, it's a possibility that many returned to Earth.