Because fireforming brass is a pain in the ass to most people Jerry. Custom made dies for the 6.5 Watchamacallit are generall expensive as well. Would you seriously want to waste bore life for the sake of fireforming a couple hundred piees of brass for a field rifle? Nope! Didn't think so.
Apparently, you don't wildcat much. First off, you don't need to waste ANY bullets to fireform. I use cornmeal over a light pistol powder charge. You could also fire factory cases in a true AI chamber and enjoy the shooting WHILE fireforming.
If done properly, you can use off the shelf dies to work with just about any wildcat. I use no custom dies for my 6.5 Mystic - all OTC dies. However, I may get a custom set from Lee for a whopping $55.
In many cases, the new chic low volume die sets are MORE expensive then putting together a set from common high volume cartridges.
As to fireforming, I do this for ALL rifles in ANY chambering. I have found that this gives the best shooting ammo. So whether I am mucking with a 223 or 6.5 Mystic, fireforming is the first thing I do with the brass. Really helps with long case life too.
For a field rifle (except a varminter), would anyone need more then 50cases? I think for many hunting rifles and most shooters, 20 cases is plenty. With this few cases, fire away.
Love to know who works up a few hundred cases for the hunting rifle
If you reload cases more then twice, you are already dealing with fireformed cases so a wildcat is of little extra work.
Yes, some consider it a pain. Others consider it a worthwhile endeavor. It is often the only way to unique solutions for specialty rifle endeavors.
YMMV.
Jerry