Why do scientists hypothesize that chemical reactions and molecular activity preceded life on earth?

The chemicals of life must have preceded the formation of life on earth. This is especially true in the formation of organic molecules especially DNA and RNA and amino acids. Spontaneous reactions in the oceans from chemicals from methane, water, hydrogen, ammonia, and etcetera, occurred in the waters. Energy from the sun and lightning strikes spurred these spontaneous reactions. These organic compounds in the waters when aggregated underwent formations to form photobionts, the precursors of life.