Here's why:

* Formation: Sedimentary rocks are formed from the accumulation and cementation of sediments, which are fragments of pre-existing rocks, minerals, or organic matter.

* Grain Size and Shape: The individual grains in sedimentary rocks are generally not perfectly shaped or sized to fit together tightly. They often have irregular shapes and sizes, leaving spaces between them.

* Cementation: The spaces between the grains are filled with a cementing material, usually minerals like calcite or silica. This cement binds the grains together, but it doesn't create a perfectly interlocking structure.

Contrast with Igneous Rocks: In contrast, igneous rocks, which solidify from molten material, often have interlocking crystals due to the way they cool and crystallize.