From as early as the late 1970s, cyberpunk writers were describing the use of a computer as being an avatar traversing a physical landscape. This was for multiple reasons.
Firstly, and quite simply, most of these writers were imagining the use of future computers, and had no idea how the technology would develop.
Secondly, computer usage is somewhat intangible, (especially so for previous, less tech-literate generations of readers) so writers needed something tangible to describe besides the clicking and typing.
So a file becomes a room, a database becomes a building, password protection becomes a locked door, hacking becomes burglary, different websites become the various neon-hued districts of a city. The most powerful entities have skyscrapers while the average joe lives in the slums.
The common 'data city' was probably so popular because cyberpunk emerged when gigantic cities and corporate skyscrapers symbolised power and was where you would find these computer networks being described. They were very urban, and not yet found so ubiquitously in the home and school as they are now.
It was not long before this idea became outdated. Today, users barely notice the UI and its structure, often have no avatar besides perhaps a username (which is often just a real-life name), and would never imagine traversing a landscape in order to move from one For You page to another. The data city became a poetic idea from an older time.
Then in 2022, Mark Zuckerberg had the same idea and called it the metaverse. No one has any idea why.