Question

why are seeds covered by soil after sowing

Answer 1

Many annuals, wildflowers, and vegetables can be seeded directly in the garden, either broadcast over a bed to give a planted-by-nature look or sown in the traditional rows of a vegetable or cutting garden.

Answer 2

Sol:
Levelling of the soil is to be done before sowing the seeds. It is done after ploughing or tilling procedure. Levelling increases the water holding capacity of the soil. 
Seeds are sown after levelling. These seeds must be covered with the soil to prevent from being blown away by aind, picked away by birds, eaten away by animals etc.