The '5 number summary' was originally developed by John Tukey as part of his regular data analysis in the days before computer graphics. He later developed the 'box and whiskers plot' as the graphical representation of the same summary. So JMP is merely following the historical practice that is used by many analysts.
Your situation might be addressed by adding a plot of N for each group. That result is easily accomplished with Graph Builder:
Or perhaps include the markers for the data to get a sense of the group sizes.
The best solution depends on the particular details of your data, of course.