Darwin's theory didn't show how the variations occur or what was the basis of the variations occurring in the organism. He failed to mention the concepts of DNA. He failed to mention the origin and transmission of variations in his theory of evolution. The Oparin-Haldane hypothesis suggests that life arose gradually from inorganic molecules, with “building blocks” like amino acids forming first and then combining to make complex polymers. I don’t know whether Oparin dealt with this issue, but I could think of several reasons. For example:
The chemical make-up on Earth today is not the same as it was 4 billion years ago. There are several differences, but one especially important factor is that the primitive atmosphere was almost devoid of free oxygen, which would have been harmful to early life that wasn’t very developed yet.
Any life that arises today would also have to compete with existing life, which is already much more advanced.
It’s also possible that some life does arise, say, every few million years… but that we haven’t observed it happening.
You need to understand that we don’t know yet how likely it is for life to appear naturally… and what the exact conditions are for this to happen.