For a game with a scope as immense as Starfield, there should be at least a subsection of the community actively discussing the lore and any related fan theories. This is, at least in other games, the most lively part of the player base.
One Redditor noticed a significant lack of such in Bethesda’s newest open-world RPG and established a thread asking the community what they think.
While there is the now-ubiquitous comment about how soulless Bethesda’s writing is, some injected theories on why nobody had ever returned to Earth to recolonize it after a couple of colonies.
User Technogen explained that, at some point, the lore revealed that the United Colonies, who maintain a presence in and around Mars, are keeping other freelance travelers away from the planet.
When someone pointed out that humanity has spread itself out to even planets with no atmosphere like Earth, other users pointed out that only outposts survive on those planets, not cities.
Case in point - highly concentrated populations and cities are on planets with Earth-like atmospheric conditions like Akila and Jemison.
However, there is a common consensus among the Redditors that Starfield’s writing is so close-ended that it is practically impossible for any theory or theories to develop, even if the galaxy is that immense. Whatever lore-building or discussion about Starfield is in a separate subreddit.
Whatever additions to the lore we can expect will certainly come from the future DLC, Shattered Space. Unfortunately, the information on that additional content is sparse as of writing.