And Native Americans prior to 1492 might have thought sailing across the ocean was impossible. I can't agree that because we have no evidence that warp drive has been used already means it's impossible.
Furthermore, I doubt there are as many "civilizations" in the galaxy as you seem to believe.
-Of all the planets out there, only a fraction support life (as we understand it)
-Of those that support life, only an INFINITESIMALLY small fraction would take the evolutionary route toward intelligence
-And of all intelligent life, I doubt many would feel the need to use tools and develop technology (look how happy dolphins are without warp drive).