For a few reasons.
1.) Its irresponsible. In the sense you're using up a very finite, rapidly depleting, national resource without any accountability. In most developed countries, its illegal to tap directly into the water bed, especially without a gauge measuring the amount of water you pump up. I know its convenient, but as I said it is highly irresponsible, for you, the countyr, and future generations.
2.) Karachi is so close to the sea, that our water bed has a very high salinity. It would need to go through a very expensive reverse osmosis process before we can actually use it in our homes.