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.

I think that a clarification is in order. When a switch, relay, transistor, etc. is NO or NC, it is in this state/condition, when the device is not activated. What this means, is that a NO switch (for example) will be closed when it is activated (a magnet is in proximity; door closed). With this kind of switch, you make a "closed loop," which will be "sensed" if the loop is broken (the door opened; wire cut). This type of application, is the reason why the switch has to be NO, when not activated!

Normally Open magnetic switches can be put them in parallel rather than series when you want one wire pair to monitor several openings.

Unfortunately if the wire is cut, it doesn't alarm, but this is taken care of by having each switch in parallel with a large resistor. If the wire is cut, the resistance changes upward, and you can tell that the wire was cut, rather than a door opening (which reduces the total resistance to zero) or closed (which has a maximum resistance based on the number of openings in the string).

It's a different topology, and is more useful when you are mixing sensor types. For instance, a floor pressure switch is normally open, and closes when someone steps on it. If you wanted a door and a floor switch to be on the same circuit, it's easier and cheaper to use an NO switch at the door, than to find a NC floor pressure pad.

Most alarm systems today have enough zones, though, that mixing various types in one zone isn't needed.