I really don't buy the claim that D&D has always encouraged worldbuilding. Or at least not to the same degree as we now thing of conworlding.
There are a LOT of flavour assumptions in the 1st Edition PHB. I'd recommend people take a good long look at those books without the lens of nostalgia. Actually read the early D&D books again. They can be surprisingly vague and uninformative at times.
For example, dwarves hate orcs, gnomes hate goblins and kobolds, orcs can breed with humans, which races are adept at which classes, demihumans are less proficient than humans, Vancian spellcasting, the gods exist and grant their worshipers superpowers, wizards like Rary and Bigby exist, etc.
Meanwhile, the DMG is pretty silent on worldbuilding. It goes into some details starting on page 86, but much of this is equally on structuring a campaign as worldbuilding. And really offers to practical advice (and advocates creating a bottom-up world starting with a village and dungeon).
And most of the old "modular" adventures assumed Greyhawk. They have a set location in their backstory. It's less that they were designed to be modular and easily dropped into peoples games and more that stories back then were just simpler and location based, making it easier to move locations and change details. Because the world didn't matter and was just a location to house dungeons.
The current edition offers a wealth of information and advice on building a world. An order of magnitude more than in the 1e books. It's not that earlier editions encouraged you to create a world, they just offered less of a choice.
The 5e books kinda present the Forgotten Realms as the default. But it's much less of a default than Greyhawk was in 3e/1e and far, far less than the Nentir Vale was in 4e. There are alternate gods included and mentions of other worlds in the flavour text of races. But the intent is obvious: people uninterested in worldbuilding are presented options to play in the Realms or play in Greyhawk. And having that choice is lovely.
Not that I'm against worldbuilding. I literally wrote a
book on the subject. I was working on the map to
my world just before stopping to write this.
I'm just in favour of people having the choice. And having as much information as possible if they choose to make their own world rather than just being told to do it.