Seems to me that long ago replica muzzleloaders started flooding into America. Most were European made and very cheap in cost.
If a breech plug was made coned, it is probably for the same reason that many other various parts on various replica guns either worked miserably or did not work well at all, for a high proportion of the sales. THE MAKERS JUST DID NOT KNOW ANY BETTER.
These guns were sold on the cheap because they were.