Because being dogmatic is exactly what causes people to start ignoring this sort of methodology. Pragmatism really does have to win out sometimes. Maybe you're in a hurry because the bug is causing active downtime. Some bugs really are "obvious" once they've failed and you're looking at the code. Maybe development of a proper test involves some test infrastructure work that's a larger undertaking than you have the opportunity for at the moment. Maybe you have a solid manual/QA testing system behind you, allowing you at least temporary assurance that your fix is valid.

No team does everything perfectly all the time, and that's fine. The real question is what gets done about it afterward: is the technical debt that you've incurred paid down in a reasonable time frame?