I think you're misinformed. People sleep under tarps and tarp shelters all the time. You have to be a bit more selective about your location, but not much. For instance, if you're in a large depression in the ground when it rains it doesn't matter if you're under a tarp or in a tent you're still going to get wet if it rains enough. The only time a tent might really save you on that score is if you're setup on a minor slope where a small amount of water will roll through. You also need to think more about wind.

Still it's a perfectly reasonable choice for folks with some benefits like weight, better ventilation, and more.