Star Trek is about flying around in space and encountering weird shit. Dudes stood around on Earth chatting is the absolute opposite of everything I want to see in Trek.
I think many would disagree - Trek at it's heart, good Trek anyway is using "aliens" to hold a mirror up to real world issues. Trek at it's best is stuff like "Let that be your last Battlefield".
Earth is the most important diplomatic hub in the Federation - it's at the very heart of everything.
All the alien issues, the morality plays can come to you as much as you can go to them.
What's the real world issues we're dealing with today? Degrading Living Standards, Terrorism, Rogue states with super weapons, failed states and political vacuums leading to extremism.
So much stuff that can be tackled in a sci-fi context