Which isn't really what people mean when they say "toolkit".
The Core Rulebook is a toolkit for Pathfinder. It still has a default assumption in Golarion. You can strip that out and use PF to run any setting you want (even D&D settings) but in absence of any specific setting, you default to Golarion. Everything in PF is written to fit in Golarion, regardless if you want to use it somewhere else as well.
What does D&D default to? Depends on the edition. Basic had the Known World/Mystara. 4e had Nerath. 3e (and to a degree, 1e) had Greyhawk. 5e used the Forgotten Realms for a while but claimed it was the Multiverse. 2e is the only one who used no specific setting and surprise surprise the core rulebooks are some of the worst for invoking the feel of D&D.
I keep hearing the 5.2 books will have a chapter with a default setting (odds are Greyhawk). I want that spread to the remainder of the core books. Use the Oerth elf lore as the PHB elf lore (and imply other elves are different). Use the Greyhawk Gods as the example pantheon. Every piece of lore in the core should be ready to go in a Greyhawk game without adaptation and if you want to run a different setting, you rely on that book to make the changes.
(And if Oerth isn't the DMG default, replace all above with whatever setting is there).
The Core books should allow you to play a complete D&D game with no additional world building or campaign setting books needed. The fact that D&D charges nearly $200 for its core rules and still requires the DM to buy or do world building is a crime. Give a single example and let that lore be the default unless the DM wants to change it.
