Because until the manual starnut installation is eliminated and companies start preinstalling to a default position, it's a huge hassle for riders who don't know how to install it. I see people having to install two or three at a time because of the starnuts being put in too far into their bars.
The way I see it, we are FAR behind the times. A bar should come stock with a preinstalled starnut, exactly how a BMX fork comes with one pressed and welded in place. Making it easier on the person installing it.
But aside from that, it's also just a dumb idea when we have fork threads becoming thicker and thicker. Once you thicken the tubing, there won't be any stress to the threads. And even if there is, it's minimal and the fork will still last you months, if not, years. Compared to a stock Pro Model fork.