Consider that every time you start a session with Claude Code. It's effectively a new engineer. The system doesn't learn like a real person does, so for it to improve over time you need to manually record the insights that for a normal human would be integrated by the natural learning process.
Yes, that's exactly the problem. There's good reasons why any particular team doesn't onboard new engineers each day, going all the way back to Fred Brooks and "adding more people to a late project makes it later".