Why only in the XVIII century. the first scientific assumptions about the origin of the earth?

Because until this time there was little information about the planets and the sun. Only by the 18th century science has accumulated enough information about our planet and the solar system.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.