Even the slant 6 cams are ground on cores from 106-109 LSA for whatever that's worth. My thinking on it is, the factories wanted a good vacuum signal for vacuum accessories and good street manners.
That’s a big part of it. Also, you can add to that the added complexity of getting a carb to work with tighter LSA’s. The factory doesn’t care about power at WOT. And that’s what the dyno is measuring. WOT. Not at part throttle, low load.