Partly because of the way some are made.
Most are ceramic not unlike clay brick or terracotta pottery.
It is difficult to classify, enhance and fire clay to be uniform
and homogeneous. It's less difficult to low fire or sun dry a central
run of the mill core and then coat that with slip or better clay and
press that into a final mold. There is no way to know anything about the
internal structure until you lap the outside away or a chip exposes
material for inspection.
I do have one barber hone that I wish had never been lapped flat.
The non label side has a soft gritty ill fired center that does not
hone a razor while the face does an OK job. The soft gritty
center is almost sandy and is not kind to a razor.