Until you ask someone with basic knowledge of feline physiology.

…Cats are intentionally promiscuous and can have litters from multiple fathers. It won’t change a thing. Unless you sterilize almost all male cats. Before that it doesn’t change much - and as vasectomies are far more cost intensive you can actually sterilize less of them. So more fertile male cats will stay in the population.

Besides it is actually kind of cruel as a significant portion of non-castrated feral male cats get injured during these fights - so it adds an unnecessary layer of suffering as well.