@allenJo - That’s an excellent question. I suppose the answer would be, in a word, precision.
Subtracting out the width of the rim would yield an approximate number at best, and it appears from the article that you need these devices for spot on precision.
Also, you assume that there is always a rim with a clearly defined width. What if all you had was a hole in a piece of wood, rather than a ring with a clearly defined rim?
Where would the outside diameter begin? In short, I think for engineering purposes the inside micrometer is the only tool for the job.