Because this is what my manual says. I have never heard of having to do this before. Change the pressure if you're transporting something heavy or check it if you're going on a long trip, yes. But I didn't know you have to check it once a month. I've never seen my parents do this either.
what else don't I know 
YABU: everyone knows this, you idiot!
YANBU: I didn't know either and don't check regularly