Lots and Lots of Water in the Atmosphere!

Total moisture in the atmosphere
In total there exists, at any one moment, 12,900 cubic kilometers of water in the atmosphere. 
Liquid water in air
98% of this moisture exists as vapor while 2% exists as liquid i.e. there are:
~ 258 cubic kilometers of water in liquid form in the atmosphere, and
~ 12,642 cubic kilometers of water in vapor form in the atmosphere
Using 1 cubic kilometer = 2.64172052 × 10¹¹ US gallons, there are:
~ 681.56 x 10¹¹ US gallons of water in liquid form in the atmosphere, and
~ 33,396.63 x 10¹¹ US gallons of water in vapor form in the atmosphere
Moisture in air replenishment rate
All the moisture in the atmosphere is, however, replaced every 9 days i.e. 12,900 cubic kilometers of water is delivered to the surface of the Earth from the atmosphere every 9 days i.e.
~ 681.56 x 10¹¹ US gallons of liquid water falls on Earth every 9 days, and
~ 33,396.63 x 10¹¹ US gallons of vapor water falls on Earth every 9 days
Water deposited on Earth from the atmosphere every day
~ 3,787 x 10¹¹ US gallons every day
For a 10 billion human population, this is ~ 37,870 US gallons/person/day
What's behind global water scarcity?
Recognizing that ~ 37,870 US gallons/person/day is just NEW water and does not include water from rivers, lakes, underground aquifers and other sources, the question arises:
Why do we have a water scarcity issue in most places around the globe, today?
Granted, we cannot capture every, or even most, drops for humanity's use, water scarcity remains a challenge for human ingenuity in two ways: How do we capture precipitation in locations that nature chooses and how do we transport the collected water to where we want or desperately needs to have it?