There is another side to that equation as well.
High in the atmosphere, water molecules are broken down into their constituent hydrogen and oxygen.
The hydrogen, which is lighter and faster, has more of a tendency to escape into space, leaving the monatomic oxygen atoms behind.
True. It is a very complex thing, and most importantly, it is DESIGNED to be self-balancing. We only seem to accumulate as much ‘water’ as is needed. The ‘excess’ hydrogen is vented to space, and the oxygen is turned into ozone by the energy from the Sun. Nothing is wasted, everything has a purpose.