The reason nobody offers pre-fit sporter barrels is because the barrel market is always flooded with factory take-off sporter barrels that have consistently proven to shoot 1/2 to 3/4 MOA with good handloads, and such barrels can typically be had for less than $100. Why spend $200-400 on an aftermarket barrel for a hunting rifle when a factory sporter barrel can be had for much less and is more than accurate enough for hunting purposes? Only real reason to go aftermarket on a sporter barrel is if you want it chambered for a cartridge that isn't available from the factory.

The barrel makers and brokers know that 95%+ of those looking for a pre-fit aftermarket barrel are going to be wanting a heavy barrel for long-range varmint hunting or competitive shooting so it makes no sense for them to tie up their money in stocking sporter weight barrels that are just going to sit there and collect dust until a rare individual like yourself comes along. For every sporter barrel they sell, they'll sell 100 heavy barrels.

Now factor in all the different cartridges - what maker or broker wants to tie up his money in dozens of pre-fit sporter barrels in various lengths and cartridges for a once in a blue moon sale of one?