Hydrogen is not present in the atmosphere in bulk (the original question stated it was); if it were, when you lit a match the air around you would explode in flame. Most of the hydrogen in air is in the form of water vapor. Water is H2O - you can't make water with only oxygen. There is way way more water vapor in a typical atmosphere than there is free hydrogen.
It is just extremely more practical to get water from the ground, or in some rare cases, from the atmosphere. But there is also no practical way for a plant to synthesize water from the atmosphere - plants need to use a LOT of water, way more than things like nitrogen, potassium, etc, and the ingredients just aren't there.
To clarify a bit though, there are reactions in plants that produce water: plants use cellular respiration just like animals do, but the source of this water is oxygen from the atmosphere plus sugars synthesized by the plant - those sugars originally required intake of water, so there is no net production of water for the plant.
It isn't an issue of plants "wanting" to do anything. The big thing that plants make for themselves (that animals do not) is by their ability to turn atmospheric carbon into sugars and carbohydrates via photosynthesis. Everything else is either A) not particularly distinct from other organisms, or B) depends on the plant species. Even nitrogen fixation isn't really done by plants, but by microorganisms, some of which have symbiotic relationships with some plants.
The general rule is that organisms have roughly 3 ways to get the things they use from the environment, in order of efficiency: diffusion of things that are in high supply in the outside environment (depends on the particular environment but might include oxygen from the air, salts in the ocean, water in some contexts), using pumps to transfer desirable things into the organism directly or indirectly by utilizing established concentration or electrical gradients, including osmosis - note that these processes occur in digestion as well, and finally, synthesis. Synthesis can be expensive, depends on the development of very specific enzymes, and requires component parts to be available, so when the other options are available there is not much selection pressure to develop synthetic machinery. Water is incredibly abundant on Earth, and even where it is rare, it is present in the environment in more accessible ways than are the ingredients.
There are some organisms that produce enough water through metabolism that they do not need to drink; plants would not be included in this category because they need to use water to produce the products that the metabolic water would come from. However, the source of water in these reactions is not gaseous hydrogen and oxygen, but produced from cellular respiration.