Inspite of long range order in arrangement of particles why are the crystals usually not perfect?

In spite of long range order in the arrangement of their constituent particles, yet crystals are not perfect. This is due to the presence of crystal defects.

The defects are basically irregularities in the arrangement of constituent particles.

There are 2 types of crystal defects.

Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance.

Line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points.