At a fundamental level, there is a difference between Greyhawk and FR as it related to lore and canon that is neither better, nor worse, but different. And, as a general rule, I think that the difference tends to appeal to different sets of people. I would further say that this difference is mostly an accident and a legacy of history- not something that was necessarily foreordained.
As I have written about before (and alluded to here), TSR (and Gygax) didn't understand that people would want pre-made adventures and settings. That's why they were happy to license that material to Judge's Guild at the beginning; the real money was in the rules (and, of course, TSR was originally set up to publish a lot of different rules for different games). So it was only somewhat belatedly that Gygax and TSR saw that people were clamoring to hand them money for adventures (modules) and a campaign setting. That's when Gygax used the (modified) version of his home setting as the first D&D official campaign setting- Greyhawk.
But there was still the core belief that DMs would just use the material and make it their own. Which is why the original Greyhawk (folio, campaign setting) is unlike more modern settings in a way that is surprising to, say, a modern FR reader- it is basically just a skeletal outline, full of ambiguities and hooks, waiting for a table to make it their own. Every Greyhawk is, essentially, different, because every table will come to different answers to the hooks provided within the setting; in addition, this was completely in keeping with the multiverse ethos at the time, which provided that there were an infinite number of Greyhawks that a person could travel to, each slightly different (in fact, this was the way that people could "port" their PCs from one campaign to the next).
The very first FR, as modelled in the Gray Box, was both different than GH, but also subtly the same in that it provided more questions than answers. To this day, that is why you see that there are people that will say, "I am an FR fan, but I like the Gray Box." In effect, they are pining for the FR version of that early GH ethos (I would add that the Gray Box also had rules for portal-ing your characters over ....). But this is where the paths in the woods diverged.
Because Gygax was ousted, Greyhawk never incorporated all that additional lore (or cruft). Sure, there were updates in the timeline, but because GH never added hundred of narrative books, and computer games, and ate up additional campaign settings, and had to keep retconning features to keep up with changes (the "spellsunderingplague") ... and because there was always some degree of flexibility regarding "canon" and "lore" because of the very nature of how it started ... it was always more open-ended.
FR, on the other hand, as I have stated ... well, the lore has lore that has lore. There are guides that go through, in detail, the levels of canon. The thing is- as a geek, I understand why that is appealing! There are many of us that love to do the deep dives into lore and canon- whether it is Star Trek, or Doctor Who, or Marvel/DC, or any one of a number of subjects that gets the blood pumping (or angered, as it seems increasingly the case). And, as you correctly note- FR, more than any other setting, has the mostest. Of all of it. Which is either a great thing, or a not great thing, depending on your approach.
There are people that totally love all that lore and canon and get into it! I think that's awesome. Others ... not so much. That's fine too.
