Because it's remarkably easier to write bugs in a code base you know nothing about, and we usually try to prevent bugs entirely, not debug them after they are found. The whole premise of what you're saying is dependent on knowing bugs exist before they hit Prod. I inherit people's legacy apps. That almost never happens.

In sufficiently complicated systems, the 10xer who knows nothing about the edge cases of state could do a lot more damage than an okay developer who knows all the gotchas. That's why someone departing a project is such a huge blow.