Why can’t we absorb oxygen from the water (H2O) we drink?
Because the oxygen atoms themselves are less important than the chemical bonds they can form.
The chemical bonds between two oxygen atoms represent a significant amount of potential energy (think of a stretched rubber band), especially compared to the chemical bonds between oxygen and hydrogen in H2O (think of a loose rubber band).
The potential energy if the O2 bonds was already released (think of pulling a stretched rubber band off of one of the pegs holding it) when it reacted with H2 to form H2O, so that energy simply is not available to be extracted from H2O
Because doing so would require multiple biological function that we do not have, partially because of how complicated it is, but also because of how damn bad it is rentability-wise and usefullness compared to our old good breathing
We actually do the exact opposite. We make water out of oxygen in the air. We do this by combining it with protons, which are really just hydrogen nuclei. That is why we breathe oxygen, to dispose of all of these waste protons that are a byproduct of us digesting food.
The reason we cannot absorb the oxygen in water is because molecular oxygen is a very reactive substance. It forms very stable bonds with protons, and it is comparatively quite hard to break those bonds apart. We don’t have the biological tools to do it. Plants do have those biological tools, and do they use them to extract the protons bound to oxygen in water and combine those with carbon dioxide to make sugars.