Unfortunately layering is not as simple as "defense in depth". Each layer may have its own vulnerabilities, and so the attack surface is increased. There is als

Unfortunately layering is not as simple as "defense in depth". Each layer may have its own vulnerabilities, and so the attack surface is increased. There is also a history of issues in mis-matches and side-channels arising in how layers are combined, which can be very subtle. So overall there's a trend to simplicity and as little code as possible.