Loose neutrals in a house can easily cause house fires and are not always detected until it is too late. Why isn’t there main “smart” breakers that not only detect loose neutrals / phase imbalance and blow main breaker.
Are you suggesting to detect a loose neutral in final subcircuits, or on the incoming mains?
Loose neutrals in final subcircuits will generally cause series arcing, which should be picked up by AFCI breakers. As the fault is confined to one circuit, there is no reason to trip the main breaker.
AFCIs aren't mandatory everywhere for cost reasons, and because they are quite prone to nuisance tripping where there is no fault.
Detecting over or undervoltage in the incoming feeder is possible. Some more critical commercial sites in areas with frequent power outages have this protection, which is usually used as a trigger to switch to generator power. Typically a separate relay would be used, which could be combined with a shunt trip device.
Loose neutrals on the feeders are not necessarily any more likely to cause fires than a loose phase. Damages are typically only to equipment overvolted.