How do we know it only happened once? Maybe it happened multiple times but only one version survived while the others were outcompeted?
We don't. But we know we can't replicate it, have never observed it, don't seem to find half-assed attempts at it in the wild and that there weren't multiple competing chemistries that found themselves co-existing, there was one.
I find it remarkable that chlamydia cells, fully enter host cells and live there stealing of resources.
I would call it evolutions “half-assed attempt” at endosymbiosis. (Disclaimer: evolution has no goal)