Kinda of suprised other companies do not include them
Many cartridges share a shell holder with other cartridges. Example: 223 Remington, 222 Remington, 222 Remington Magnum, 221 Remington Fireball, 17 Remington, 17 Fireball, 204 Ruger, 380 ACP and a bunch I do not remember at the moment.
308 Winchester/30-06 is another case head size covering many cartridges.
While these are extreme cases at one end of the spectrum and there are others where the case heads are unique, some of the manufactures have decided reloaders do not want to spend extra money for duplicate shell holders.
There are at least two schools of thought from reloaders. One, have the minimal number of shell holders and two, have a shell holder with each set of dies.
I prefer to not pay for extra stuff I do not need so I would rather not get shell holders with each set of dies. Besides, when I buy dies for my progressive press, the single stage press shell holder would be just a boat anchor, albeit a light one.