Because without this critical component of random mutations, this throwing out of the word "synthesis" can't keep the crows out. What is the synthesis? No one knows. No one can define it.
It's not some kind of secret, Dash, it's just another example of the huge amount of science you've simply failed to pay any attention to.
The "synthesis" is nothing more than the synthesis of Darwin's original theory of common descent by modification and natural selection with the modern science of molecular genetics and what we know about random mutations.
That's the synthesis. It's not some secret code word, it's just the combination of Darwin's theory from the 1800's with modern discoveries about DNA.
If neo-Darwinism can't explain it, most atheists are smart enough to know that they have nowhere else to turn.
We have everywhere else to turn. We have the entire world of biology to turn to. It's creationists like you who have a one-trick pony and nothing else to ride.