While you're not wrong, I see the weeks-long duration being quite the racket as well for magical security contractors.
Talk about a dream of a racket: Yeah, we'll have to be back next month to put your ward up again. And charge you again.
I kind of assumed that was the norm. And permanent wards were rare even for corps. Karma is too valuable to blow. Even the corp might prefer you save it. But its a meta concept so its hard to think how people treat it in the setting. Permanent ward that some fragging Runner drops vs 1/3 my way to next initiation?
Except Corps are also cost watchers.
Having to constantly pay out for travel for a bunch of mages to cast a bunch of Rituals on the same building every 2-6 weeks VS doing it once.
As in the cost of the reagents used for one casting VS casting every 3 to 6 weeks...
Add in having to have those same mages travel back when the ward is disrupted, VS the ward restoring itself the next combat turn..... (Core, pg 316)
But yes, this is very meta. but probably the best explanation for why wards are so common compared to the number of mages. (on 2% of the population is awakened remember, less then that are mages, and less again can cast the Ritual..)
As to Karma and NPCs... well.. Notice the Corps churn out a LOT of shit that requires karma somewhere in the creation process... its almost as if they have a Karma Forge somewhere.. or as much as the plot requires..