We often see cracks on rocks and stones why

Due to the process of weathering and erosion.

Explanation:

As the rocks are made of different forms of minerals they may have an igneous, metamorphic or sedimentary origin. Hence the natural stone surfaces can be made of granite, lime, or even shale.  The process of physical weathering that is called as mechanical weathering of rocks tends to break them apart and leads to the development of crack and small openings. The diurnal exposure of bare rocks to extreme temperatures of day and night leads to onion peeling and slowly with water as an agent of erosion lead to cracks in joints of rocks.