The problem is that many of the places they use infinite scrolling have such large datasets that doing this would make the scrollbar borderline useless for smal

The problem is that many of the places they use infinite scrolling have such large datasets that doing this would make the scrollbar borderline useless for small movements (move back half a page or 1-2 pages) -- which probably account for 99% of scrollbar uses in this context. Google search for "cat"... Of course if you actually advanced extremely far in a typical "infinite" scrolling view, the scrollbar would similarly become pretty useless, but in practice very few people actually scroll all that far, so it seems better to optimize for that case. They could meet halfway by making the "scrollbar readjustment" chunk size somewhat larger than the "load new data" chunk size, so you get the on-demand loading efficiency of the latter with fewer scroll-bar jumps, but I don't think they could make the former too large without creating more usage problems than they're solving.