The Browning A Bolt is a really great rifle and I would guess that the action isn't chosen for custom rifles by the makers of these gun is that the customers of these guns generally want an action as close to a pure Mauser type with claw extractor as they can get. Look at the screaming that happened when Winchester dropped the claw extractor in 1964.
I own on custom rifle that was built on a K98 Mauser action with a douglass premium barrel and a beautiful wood stock built in the early 80's. I got the rifle in an exceptionally good deal and to replace it today would cost many thousands of dollars. I had a sister gun to this one in 25-06 that I gave to my son.
I own and enjoy my stainless Browning A bolt in 30-06 with a wood stock so don't think I'm not a fan of Browning A bolt rifles.