Because living things do not come into existence without a source or progenitor, with the unique exception of the Source of all life, God. (Psalm 36:9)
So there's an exception. Any exception kills your theory. You cannot claim that nothing comes into existence without a cause, or source, and then turn around and say, "well, except for my one pet exception."
If one exception is allowed, particularly with no reason or explanation why this one exception should be allowed, this means that any exceptions can be allowed, and your original statement that nothing can come into existence without a source/cause is proven moot.
The concept of spontaneous generation was refuted long years ago.
Spontaneous generation does not equate to abiogenesis.
Spontaneous generation was the belief that an animal, like the fly, can be produced from other matter, fully formed. This was based on things like rotting meat appearing to spontaneously produce maggots. This was disproven when Pasteur showed that if you seal off the rotting meat from the environment, and don't let anything in, then it will not produce maggots (thus, the maggots must be produced by something else, and not just the meat.)
Abiogenesis is the idea that various chemical elements slowly created compounds and eventually one or more of these compounds developed the ability to replicate itself-- like DNA or RNA. This is not a difficult a thing to imagine happening if you know basic chemistry and how certain elements attract or repel each other.
Now that I've told you the difference, I hope we don't see you conflating the two anymore around these forums.