Because forging legitmate high grade pipes is incredibly difficult and time consuming and not profitable. If you stamp up a fake Bo Nordh, a dozen Bo Nordh collectors will point out all the reasons it's fake, and the pipe will sell for 90 bucks even if it took a week to make. No, you make a pipe that bears similar marks to a somewhat more expensive brand (or in this case, brands) and hope to catch someone who mistakes it for the very similar but higher priced pipe.
I'm one of I think only a handful of people in the world who could make a reasonably good fake Dunhill Shell. I've made replacement Dunhill style stems enough to do it. I've made Dunhill-type shapes and blasted them like crazy. So why don't I make fake Dunhills? Because when I make a pipe that looks as good as an old Dunhill, it gets bought in like 4 seconds. There's no point, business-wise. And if I did do a fake Dunhill outright, get stamps made up etc, there would still be a bunch of experts out there who would find some little problem with the pipes and denounce me. Let alone the steady stream of old never before seen "Dunhills" pouring out of my shop.... no that can't work. You start faking up Ilsteds or something, you get burnt fast.
The issue here isn't making forgeries. The issue is marketing pipes that look enough like other pipes, stamps and all, to catch the interest of potential buyers at a certain price point.