This may be the simplest answer.

We may just not have been able to observe things long enough to make the determination of how the initial event took place. Even if we ran tests in the perfect environment and were able to replicate those tests 1000 times faster, it would take us a million years to see a single success that took a billion years. What if it happens more than just once (say, a thousand times) in that time period? That's still 1000 years to observe success.

It wasn't until the late 19th century that we could even observe cellular division, so at best we've been at it for 150 years. Not to mention that we haven't been looking/testing for it for very long. We may have a long way to go.