I think we agree pay for lower tier and expect it. Pay for higher tier and require it.
I do believe you could take a gun with good parts (meets or exceeds spec) and take the time to QC the assembly yourself before you fire a bullet and you will have something. But that’s the thing most users don’t do that, and if it’s a more expensive rifle all you should have to do is clean, lube, shoot, repeat.
You absolutely can, to an extent. Some parts require total replacement when they are out of spec or manufactured improperly.
I'd be surprised how many "builders" have a torque wrench, let alone proper gauges and fixtures.