Why our senses are not sensitive enough to apperceive tiny changes in the environment? e.g. a single atom hitting the skin.
 
In fact, we do not need to be so sensitive, and even not allow to be so sensitive under the basic laws of physics. Senses are used to monitor the changes occurring in the environment, thus making response to adjust ourselves based on the trends, as well as the simplest organisms. One can not predict trends precisely by inspecting one single atom, as in micro scale a atom’s movement is unpredictable due to its thermal movement. Even a bigger molecule that contains hundreds of atoms is not trustful due to its brown movement. Trend is meaningful only when there are lots of atoms or molecules, and can be detected after counteracting individuals’ thermal movement and brown movement.
 
Technically, we are too large, compared to a single atom, to feel the movement of a single atom. In other words, to measure tiny things’ movement we need tiny scales; but shall not be so tiny that the thermal movement and brown movement can give significant effects, thus losing the precision. Thus a series of proper senses are necessary to be developed.
 
Luckily, we do not need to worry about this problem, because we have been designed well enough. Even more surprisedly, "Demiurge" endows us with a powerful brain to understand the world.
 
(Note, the discussion above do not apply when the sense is to monitor existing of something, ie. whether a element exist or not.)