Why would you assume that? That's like saying by the time we can manufacture a better engine we'll be able to replace a running one with the new design.

For example, evolution has optimized and delivered a mechanism for turning gene edits in a fertilized egg into a developed brain. It has not done the same for incorporating after-the-fact edits into an existing brain. So in the adult case we have to do an extra giga-evolve-years of optimization before it works.