That decision would be way over my pay grade but it's likely due to the bean counters and cost analysis. Copper core plugs will generally go 24k miles with no problem. Around the 30-40k mark they can be a problem. Plat. and Irid. plugs will generally go 40k and up without a hitch. In either case subtle misfires can develop that may not be noticeable to the driver and there may not be a misfire code set.
My feeling anyway is that the bean counters weigh the price of the plugs against the potential cost of replacing a certain number that may fail and gamble on the cheapest way out.
I have no idea at all what car makers pay for plugs but one can safely assume it's dirt cheap. The P and I plugs would be more costly than CC but they're still cheap.
Knowing what it cost to produce a part is something I've not been involved with. The bulk of my knowledge has been dealing with warranty claims which involve dealer cost only. Under warranty the dealer is usually reimbursed cost + 25%. The dealer cost though is generally way over what it costs the factory to produce the parts.
The one glaring example I can think of that I stumbled across by accident involved a Subaru part which became part of a campaign, or voluntary recall. The dealer cost was 65 dollars with a customer retail of 98. This meant a warranty claim paid 65 + 25%. After this campaign snowballed quite a bit Subaru decided to shave that dealer cost down to the actual production cost which was a whopping 6.50. This meant the dealers made roughly 1.60 for processing this part; not a huge sum considering the man hours involved in the processing of that claim. Labor was just as bad; .2 hours (12 minutes) at a discounted labor rate.