Because studies that controlled for focus distance (ie compared children reading indoors v children reading outdoors) seemed to show that focus distance did not have an impact.
Late reply, but most of the pro-focusing-distance crowd agrees that it’s time in peak sun, plus a fairly small number of daily “reps” of near-far focusing that is sufficient. In other words, kids could spend 99% of the time outdoors just reading a book, and the handful of glances at a tree in the distance, stop sign, bird, etc would be sufficient. Furthermore, as I mentioned to a sibling comment, even if it is just the bright light, with no influence from the focusing (which I doubt!) the only practical way to get that light is outdoors.
