Because it's completely unnecessary and it's a huge motherfucker to remove.
Add in some kind of lube and the huge motherfucker becomes a complete, ginormous pain in the fucking ass that requires a stout bench, a big cheater bar and a fat friend to help you break it loose.
The last barrel I installed for a friend went on at 125lb/ft per his request.
I couldn't get it to come loose with an internal action wrench and a 20" bar.
Thanks to
@lash I had his external wrench at my house. I was able to get that fucking barrel off.
It got a good cleaning and went back on at 80lb/ft.