Hi Andee,
The main reason is pedagogical. Having a count axis can distract students from the shape of the distribution which is generally what we want them to focus on.
Another reason is that the number of dots in a stack is so dependent on the size of the dots and the resolution along the axis. I suppose there may be situations in which this number is useful, but I’ve never encountered one that made me think it was worth adding the widgetry and corresponding distraction. Interestingly, in 5 years of collaboration with projects that could have asked for it, not one has done so.
BTW Fathom doesn’t show a count axis. (TinkerPlots does.) But, of course, in Fathom you can get a histogram with count axis.
Bill