Why are some generic door alarm magnetic switches built using Normally Open reed switches instead of NC type?
N/C magnetic switches are unfortunately nearly useless for security. Since their "unalarmed" state is for them to be open, if the cable going to them is cut then the alarm will not trigger, and of course not be triggerable anymore.

With a N/O magnetic switch the circuit is closed in the unalarmed state, and cutting the cable will trigger the alarm.