i think the steam hardware survey records all gpu's it can identify, doesn't it? so if i take the test on my home machine it will report an Intel HD Graphics 3000 (I think) and a Radeon R9 280 X. it won't be possible to tell from my survey result that the intel HD graphics shouldn't contribute at all - it's simply 'there'. i suspect the high incidence of intel hd graphics might be coming from this. however, i would never be so presumptuous as to assume that its definitely the answer.
also, very hard to gauge the average pc gamer. star citizen did a survey among their backers which showed 82% build. i doubt a typical pc gamer out there would have seen the PS4 release, and then immediately bought a >1.84 TF GPU.
what's the impetus for a GPU change? well it's probably going to be a demanding game, or perhaps the release of new graphics hardware (coming later this year).