Not really though. Earth definitely has powerfull defense systems that are
not starships, like the drones in "Best of both worlds" or the giant lasers in ENTs "Demon/Terra Prime", or satellite networks catching cloaked ships that are mentioned when they
don't have them in far out space.
It's just that the
threats in all these cases were so massive and powerfull - Borg cubes, the whale probe which deactivated
all technology, the unstoppable V'Ger - that we never saw them working. The Enterprise wasn't saving the day because it was the most powerfull defense system - but because it was carying the smartest people directly to the puzzle.
I blame that assertion of defenseless planets on Star Trek squarely on Deep Sapce 9 - because it completely squandered Trek's rules of combat, and instead introduced WWII battle rules (because, honestly, they
look cooler) - in which suddenly aircraft carriers were the only direct method of defense for the mainland. And then
Star Trek (2009) re-inforcing that idea, with both Vulcan and Earth not being able to put up ANY type of resistance against a future Romulan oil-tanker/refinery vessel (

).