Why does a factory reset keep the operating system updated? I recently did a factory reset to my phone and to my surprise, the operating system remained to the last updated version (6.0.0) and not the version I had when I bought the phone (5.0.0). 

Why does this happen? Doesn't a factory reset reset the operating system too?
System updates overwrite /system. Factory reset wipes /data and /cache but doesn't touch /system. Simple as that.