I think it's because, so far, the reach of ad-blocking is quite small so there hasn't been a significant need to invest in anti-blocking measures. An arms race

I think it's because, so far, the reach of ad-blocking is quite small so there hasn't been a significant need to invest in anti-blocking measures. An arms race in blocking ads designed not to be blocked would be interesting to watch. While it would be very easy to introduce text/images/video in such a way that hides the fact it's an advert (there will be heuristics, of course); I suspect it'll be more difficult to measure the viewership/click-through-rate of such ads, as that's what will give the game away and make signal the presence of an ad. Ultimately it might come down to old-fashioned offline measures, similar to how billboards/magazines use third-parties to measure the reach of an advert without any tracking on the advert itself. This would make small content producers suffer the most. While everyone knows and trusts the reach of a newspaper site, a blog that suddenly gets 20,000 HN visitors in one afternoon, what's that worth?