Because people want to separate Greyhawk from Forgotten Realms, often because they find Faerun is the popular sellout setting that has everything. So to make Greyhawk something other than the older forgotten child, they have taken two small elements of the setting (Gary's love of S&S and the fact it was developed early into D&D life) and spun that into its personality. People want to emphasize that it's the old setting with old sensibilities rooted in the traditions of AD&D to contrast it with a gonzo, anything goes Forgotten Realms. Both are mischaractetizations of the settings but it's easier than saying Faerun got popular due to TSR pushing it with various media while Greyhawk was de-emphasized after Gary's ouster.
The difference between Faerun and Oerth is small. At heart, they are both kitchen sink settings that don't deviate from the D&D expectations. The difference is scope and small amounts of lore and tone.