Why would I have to buy a whole new rifle as all the work is done to the barrel not the action.
Fair question, and I will try to share my opinion based off of doing several "custom builds" with Remington 700 actions over the years.
Please bear in mind that the Remington receivers are mass produced, and the chances of them being as square to a aftermarket action extremely slim to none. So, most gunsmiths will square the receiver face, and the bolt face, nose and locking lugs (more steps are frequently done too), as well as the recoil lug to try to eliminate the stacking of tolerances.
It's not always done just to improve accuracy, but also to improve the the overall feel of the action. Most 700 actions also come with 6-48 holes tapped for the scope base, so it is pretty common to get those drilled out to 8-40...which basically gives you a little bigger screws to use to secure your scope rail on top of your receiver. Same with the recoil lug, which will frequently be replaced with something thicker if it isn't just trued.
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Having a barrel cut and rechambered is basically the same cost as having a new aftermarket barrel chambered, but if you retain the OEM barrel, you are again using something that is mass-produced. That doesn't mean Remington barrels are guaranteed not to shoot. I could usually get mine to shoot pretty well with handloads. I'm simply stating that it is far easier to get a good aftermarket barrel to shoot well, foul less, and virtually eliminate that 'cold bore shift' of the first shot (especially when combined with a trued/aftermarket action).
By the time most guys get a 700 action blueprinted and barrel chambered (or rechambered in your case) you'll be out $500 - 800 dependingon how much you have done. Throw in the cost of purchasing said factory barreled action or rifle, and you're really close to the cost of a decent aftermarket action and barrel...which is why most guys go the latter route these days.
Adding here that factory stocks rarely fit a shooter as well as a good aftermarket one (or chassis), and those are frequently the first thing to get upgraded. By the time it is all said and done, most have invested enough to have had a custom rifle built. Your intended approach was a very common one 20 years ago, but with the abundance of aftermarket 'easy buttons', we can now assemble stuff that suits us better in the comfort of our own garages.
I don't know if anything that I've said makes sense to you or not, but I wish you the best of luck in whatever route you go.