I think the relevant part of the doc is

%   Particularly, a number of broken or ugly design sizes are no
%   longer used, the look of the bold sans serif typeface at large
%   sizes is considerably improved, and mismatches between the text
%   fonts and the corresponding math fonts are avoided.  As a side
%   effect, PostScript and PDF documents may become smaller, because
%   fewer fonts need to be embedded.

So while like any design choice it could be questioned, I think the
fact that it does not use all the provided design-size fonts is
certainly deliberate. (I assume the above is  Walter Schmidt's original
documentation text, but I didn't check just now.)