Just off the top of my head, I'd say that they think they've never had such a child because they are probably not used to really "seeing" 2E kids. They're there, just hidden from view. It's only when their giftedness is enough to compensate for and surpass their special needs that anyone notices. So, that's why only the more rare kid gets noticed. HG+ kids are rare enough, but throw in the 2E and they're even harder to see.
Take my dd for example. She's only in the MG range, a far cry from HG. In regular public school K, she performed completely "average" according to the teacher. (Actually, she was performing below grade level for reading but somehow the teacher didn't even figure that out.) Likewise, an HG+ kid's gifts could be well hidden by their special needs, and maybe they merely seem "bright".
