This is basically the reason we can't get rid of it. Even if we had a perfect human vaccine that blocked transmission, it wouldn't help.
As a corollary, there are unhappy accidents, and with respect to life forms in a chaotic system, such accidents can perpetuate and endure without human recognition.
The existence of gain of function research shows that you are wrong.
The very first synthetic virus created in 2002 and was modeled after polio, which is fairly transmissible and affects humans. That virus was made 20 years ago; synthetic biology has come a very long way since then.
Does that fact alter your opinion?
Synthetic biology (the actual synthesis of DNA) has come a long way, we don't understand all the components yet though.