Correct, it wasn't stolen, it was just standardized by GAP in cooperation with Hornady.
GAP had their own 6.5 GAP4s wildcat but it had a rebated rim. Hornady wanted to go with an unbelted non rebated rim cartridge for better feeding.
This is the case with pretty much all rounds. You get to a certain point and the law of diminishing returns goes into overdrive meaning there will be less and less gain in velocity for the same increase in powder.