If too many people start doing it, it becomes an issue of overdrawing groundwater. This leads to an arms race of digging ever deeper wells to reach the remaining groundwater, and on top of that starts leading to ground subsidence as ground formerly full of water dries out and compacts.

Off the top of my head, there is massive groundwater subsidence in the Central Valley of California, Mexico City, and Jakarta due to this issue, at rates measured in feet per year. This makes the land prone to flooding, and also damages buildings and infrastructure since the subsidence is generally not evenly spread out.