Why is the most significant greenhouse gas water vapor and not CO₂?

Water vapor is not officially considered to be a cause of man-made global warming because it is abundant and CO₂ is much more convenient to measure, regulate and tax. The fact that water vapor does not persist in the atmosphere for more than a few days, makes its concentrations highly variable and harder to quantify on a global scale without good quality weather networks everywhere. Its concentrations can reach 100 times that of CO₂ and its ability to trap heat is double. Additionally, fog and clouds reflect and absorb immense amounts of incoming solar radiation and moderate climatic temperatures. The water cycle reduces temperature extremes and redistributes heat throughout earth’s climates.