Standard patient monitoring indirectly monitors for blood loss. Decreased blood pressure, increased breathing rate, increased heart rate are all signs of blood loss. This can be and is done by machines.

The thing people don't seem to understand is the fact doctors also monitor consciousness. Depressed mental state is also a sign of blood loss. The patient's neurological state is important information, it has diagnostic and prognostic value.

The most basic way of evaluating a patient's neuropsychological state is to bother them with questions.