Because you can find it in "concentrated" form (think entropy), all in an aquifer or a river, and these are everywhere. But these dry up because of our usage and the climate, and when they do you still have the same amount of water on the planet, it's just not as easily accessible. It's super spread out, it's too far away, it needs a lot of processing to make usable, or all of the above.

What's cheaper and easier for you, to condense a cup of water from the air or to just turn on the faucet?