Among those who believe that life resembling the first replicator can form relatively easily given the necessary chemical precursors and environment (and cite the appearance of life on Earth just about as soon as it possibly could have as evidence for this), a common argument for why we don’t observe it is that a primordial form of life would probably be very simple and not have evolved defences against predation which would evolve as life diversified and began to compete.
Thus, they suggest, new life forms may be appearing billions of time a day on Earth, and every time it’s like “Wow! I’m alive!” followed a millisecond later by CHOMP! as some incumbent DNA-based life eats it.
It’s also possible that Earth is home to one or more “shadow biospheres” which descended from independent origins of life that we haven’t detected so far because our surveys for microbial life are mostly sensitive only to the kind of life we know about. Something that was substantially chemically different, especially if it inhabited an exotic environment such as deep underground or near a mid-ocean vent, could easily have been missed, just as extremophile bacteria were only identified in the 1970s. The possibility of a shadow biosphere was discussed here on 2022-03-25 in “Is There a Shadow Biosphere on Earth?”.