Coals occur in seams. why?

Explanation:

Coal seams are formed from dead and decaying organic matter. Generally, this matter came from ancient peat bogs that died and fell into shallow, stagnant waters. As this material decayed, it became peat over time. As this peat was buried under sediment, the temperature increased and the pressure became greater.

Coal seams originate and are formed by the consolidation of biota, minerals, and natural chemicals through geologic time. Compression, heat, sedimentation, erosion, and chemical energy are agents of the coal formation process.