The default can be a choice. A default is not "what you do when you don't want to choose" it is "the standard or baseline"
Yes, THE standard or baseline. As in, one.
I don't know what a "proper" guide would be like, but the DMG already covers religions, calendars, wilderness environments , urban environments,
governments, planar travel, creating NPCs, creating villains and creating adventure sites.
For most of those, it's in terms of how to run adventures in/with them; and handwaves at best at how to design them. Religions and governments are the only aspects they touch that involve actual worldbuilding.
A decent worldbuilding guide would have, at the barest minimum, the following (though in a more coherent sequence!):
--- tips, tricks, and how-to's on designing and drawing maps; and what to do with blanks spaces (leave 'em blank!)
--- a brief primer on medieval social geography e.g. how frequent are villages and why, the frequency and spacing of larger towns/cities, etc.
--- some ideas on species distribution e.g. a completely mixed society or each species has its own discrete enclaves or a mix of these
--- a guide to different governmental and-or social structures (including the bad ones!)
--- something about food and agriculture in terms of how much land is needed to feed how many people
--- a maybe-not-so-brief primer on weather and climate, with tips and ideas on how different world designs would alter the weather there (e.g. what if the polar axis is vertical thus there are no seasons as we know them)
--- a piece on setting astronomy and what differences would arise if the world had, say, multiple moons or two suns or was a binary planet
--- corollary to this, a guide to how tides and currents would work under different astronomies; essential if the party ever goes to sea or adventures on a coastline
--- a rough guide to ecology; just how many monsters can coexist in a given area, how do-can they interrelate, etc.
--- a rough guide to setting biology; just how are all these monsters created
--- a deep dive into how to create a coherent, if brief, history of how things came to be the way they are today
--- thoughts on how-if your world will or could connect to other worlds/planes
--- a deep dive into cosmology; what other planes are out there, what is their function, who lives there, etc. and how those planes all relate to each other
--- probably some other stuff I haven't thought of.
Note what's glaringly missing from that list: religion and deities. Those need their own separate book, in tandem with this one.