The easy answer is cost, and the main reason is standards and certification bodies.
Cost of mass produced items has to be minimized, adding a safety feature that is not required by certification and increases production costs can be seen as engineering malpractice. Cost is one of the constraints for an engineer.
Standards and certification bodies rely on engineers and subject matter experts, that generally have to come from the same industry they are regulating. They don’t want standards to become prohibitively onerous, and they have to justify and document their decisions.
Placing a 5km/hr speed limit would considerably increase safety, but would also be unnecessarily burdensome to the same society that it’s intended to protect. A cold actuarial compromise has to be used.
No system is 100% safe. The scenario you describe implies a degradation of the plug and/or socket that puts it way out of the limits of the required standard. It is a very rare scenario, that could be caught in time by an informed consumer and is commonly included in device documentation.
Consumer education (particularly lack thereof) is the main part of the equation.
There are inexpensive electronic safety devices that can be added to table saws that prevent them from slicing through your fingers. But these are not required. And consumers pay more attention to the item price than to a safety feature that dares to imply they are clumsy, that enters the capitalistic equation.
But standards and regulations can be changed. And modifications are made all the time to include new causes for concerns. It just needs people championing them. And that generally requires an informed consumer.