While some brass may not have neck wall thickness that thin, necks that's been turned may well be that thin.
There's another component dimension that has quite a range. SAAMI specs for .30-06 bullet diameters are from .306" to .309". Some company's 30 caliber bullets have been larger than .309". And die makers know that some people turn case necks to uniform their thickness so they want to ensure their dies can size those fired cases enough.
Commercial dies are made to size fired case mouths small enough to grip the smallest diameter bullet tight enough to stay in place during handling and loading. Their makers don't know what case and bullet dimensions customers will use so they try to accommodate all of them with a single die. And some combinations of cases and bullets won't end up with bullets gripped tight enough after sizing.
Reloaders have the option to get sizing dies with neck bushings or diameters correct for their cases to grip their bullets as they desire.
There's another component dimension that has quite a range. SAAMI specs for .30-06 bullet diameters are from .306" to .309". Some company's 30 caliber bullets have been larger than .309". And die makers know that some people turn case necks to uniform their thickness so they want to ensure their dies can size those fired cases enough.
Commercial dies are made to size fired case mouths small enough to grip the smallest diameter bullet tight enough to stay in place during handling and loading. Their makers don't know what case and bullet dimensions customers will use so they try to accommodate all of them with a single die. And some combinations of cases and bullets won't end up with bullets gripped tight enough after sizing.
Reloaders have the option to get sizing dies with neck bushings or diameters correct for their cases to grip their bullets as they desire.