Because Weatherby is in the business of selling you a new rifle, not fixing your old one. If they fix everybody's old guns, they don't make any big profits. And money is all they care about. Same reason they refused to even take a look at my factory-defective barrel on my Accumark .257 Wby.
Aftermarket barrel companies will do what you are asking. I'm pretty sure Bartlein, Shilen, and Pac-Nor will all do this service for you, if you request it.
But keep in mind, 2 identical barrels, will still never be identical. They might like 2 totally different bullets, 2 different powders, 2 different seating depths, etc... It's just a luck of the draw as to how similar they will be to each other.