Why couldn't earth's water have come from comets?
It could have!
It is a well-established possibility that comets were a big source of Earths water. It is very hard for scientists to prove one way or another where water on Earth came from but it is known that some comets contain significant amounts of water. So a comet landing on Earth could definitely provide water.
One theory suggests this was the case during the late heavy bombardment where Earth was being constantly hit with meteorites at a very high rate. The research on this topic is still hotly debated!