Here's why:
* The current atmosphere and environment are not conducive to abiogenesis. The early Earth had a very different atmosphere, lacking oxygen and with abundant volcanic activity. This early atmosphere was more likely to support the spontaneous formation of complex organic molecules. Today's oxygen-rich atmosphere is highly reactive and would break down any newly formed macromolecules.
* Life itself consumes the building blocks of macromolecules. Living organisms use organic molecules as their building blocks and energy sources. This means that any abiotically formed molecules would be quickly consumed or broken down by existing life.
* Complex macromolecular structures are highly improbable to form spontaneously. The formation of proteins, nucleic acids, and other biopolymers requires a specific sequence of building blocks and complex folding processes. The chances of these forming randomly in today's environment are extremely low.
The abiogenesis of life is thought to have occurred only once in Earth's history. The conditions that allowed for this event are unlikely to be replicated today.
While we can recreate the conditions of early Earth in the laboratory, and observe the formation of basic organic molecules, the spontaneous formation of complex, functional biomolecules under the current conditions on Earth is highly improbable.
