All known lofe on Earth uses the same base "toolset" to keep it simple while genetic analysis shows all known life is at least distantly related. While it isn't impossible that life developed multiple times on Earth, any other lineages were unable to compete and died off quickly.

It should also be noted how dramatically easier it is for existing life forms to hog all the available resources than it is for said resources to go through biogenesis. This is why scientists are unable to create life forms in labs, because no matter how sterile we try to make the conditions, some microbe finds a way in and thrives on the resoirce dense media that scientists are trying to recreate life with.

Biogenesis is crazy hard. Once that first lifeform appears it absorbs pretty much all available resources and takes over from there. All those conplex molecules just floating around get incorporated into the life form that appears first. Those primordial seas were constantly churning things about so life would have quickly spread across the ocean once it formed.