Because humans also make stupid random mistakes, and if your test suite and defensive practices don't catch it, the only difference is the rate of errors.
It may be that you've done the risk management, and deemed the risk acceptable (accepting the risk, in risk management terms) with human developers and that vibecoding changes the maths.
But that is still an admission that your test suite has gaping holes. If that's been allowed to happen consciously, recorded in your risk register, and you all understand the consequences, that can be entirely fine.
But the problem then isn't reflecting a problem with vibe coding, but a risk management choice you made to paper over test suite holes with an assumed level of human dilligence.
It may be that you've done the risk management, and deemed the risk acceptable (accepting the risk, in risk management terms) with human developers and that vibecoding changes the maths.
But that is still an admission that your test suite has gaping holes. If that's been allowed to happen consciously, recorded in your risk register, and you all understand the consequences, that can be entirely fine.
But the problem then isn't reflecting a problem with vibe coding, but a risk management choice you made to paper over test suite holes with an assumed level of human dilligence.