Why is the law of Biogenesis a scientific law?

Some laws are not absolute. No compound obeys the ideal gas law (although helium comes pretty close). The law of conservation of mass is violated every time matter is converted to energy. Conservation of energy is violated briefly at the quantum level.

Biogenesis implies that organisms do not appear with any detectable frequency, not "cannot ever appear". It is not impossible for life to originate, but the presence of life results in the consumption of high energy organic compounds from which life would evolve. Existing life, with billions of years of evolution behind it, makes an environment hostile to new life.