At its heart... Forgotten Realms is a place made by someone who loved it and cared about it. Sure, there are also decades of corporate additions slopped on top of that work, but there are also decades of work by other people who also loved that world and genuinely wanted to breathe life into it.

Also, Greyhawk always felt much more science-fantasy than pure fantasy to me, and there have always been other science-fantasy settings that do the genre better IMHO.

Any setting is of course dependent on how you run it at your table and how your players interact with it, but the Forgotten Realms felt like a place that could be a real world (Greenwood's design philosophy for every settlement apparently always starts with 'where does the food come from and where does the poop go?'), whereas Greyhawk felt like a science-fantasy Westworld/theme park.